Tip No. 1: Keep It Fresh : Some of TV’s most successful producers agree that even the best series can go on too long. How do some shows come back from the dead? And how do others manage to bow out on top?

On a wall of the writers’ room in the offices of the situation comedy “Home Improvement”--a wall no doubt duplicated in sitcom writers’ rooms across town--file cards list the plot lines of each episode shot during the season. As the season winds down, that wall is getting mighty full.

“But when you come back from vacation and take ‘em all down, you have this big blank wall staring at you,” says Elliot Shoenman, one of the series’ executive producers the past three seasons. “Each script is 50 pages or so, and within eight months, we’ll have to come up with 1,250 shootable pages. And when you think of it in those terms, it is really daunting.”

And when, in the case of “Home Improvement,” the series will be starting its sixth season, it’s even more daunting: How do the producers and writers keep a show fresh? How many wacky situations can one character get himself into, anyway? What can be done to keep a new episode from seeming too much like episodes from seasons past?

To sum it up: How can a program’s premise be kept from losing its novelty?

“I asked the writers that question and they said, very angrily, ‘Because we work longer hours than ever before,’ ” deadpans Matt Groening, creator of “The Simpsons.”

“An awful lot of shows stay on too long,” says Grant Tinker, former chairman of NBC and president of MTM when it was turning out series such as “The Mary Tyler Moore Show” (1970-77), “Rhoda” (1974-78) and “The Bob Newhart Show” (1972-78). “A lot of us have one drink too many at the party and become a little less scintillating.”

One man who has extended his series’ lives says, paradoxically, that it shouldn’t be done.

“If we declared a maximum of five years for every series, it would be better for the public at large,” proclaims Norman Lear, creator of “All in the Family” (1971-83), “The Jeffersons” (1975-85) and other groundbreaking sitcoms of the ‘70s and ‘80s, many of which ran for far longer than five years. “Despite the fact that the shows went on, had all those decisions been motivated by creative energies alone and not the fiscal desires of the networks, five years would be the limit for any creative effort.”

Tinker says, tongue in cheek, that all it takes is sheer perseverance, talent be damned.

“I used to say if you kept a good show on long enough, it will succeed,” he says. “There’s plenty of evidence--’Cheers,’ for sure. Any number of historic shows had slow starts, and it takes the audience a long time to catch up with them. Now, I’ve almost changed my mind. If you leave any show on long enough, it will succeed. The audience will just think, ‘It’s been on so long, it must be good.’ ”

Consistently maintaining quality on a weekly basis over a long haul “is basically an impossible task,” says Gary David Goldberg, creator of “Family Ties” (1982-89) and “Brooklyn Bridge” (1991-93). “There’s just too much work to do in a short time. You always have the episodes that don’t turn out as wonderful as you would like, the ones [on which] you would like to run a title under the action reading, ‘The entire writing staff had flu this week, and this is the best we could do.’ ”

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Which doesn’t mean that writers still don’t try. In interviews with some of television’s most successful executive producers, many said that it was that very challenge to stay creative that keeps writers and producers engaged in their work.

Carmen Finestra, another “Home Improvement” executive producer and a former supervising producer on “The Cosby Show,” says: “The first couple of seasons, you get by on the excitement. Here, none of the cast had been in a hit show. The early episodes were raw, not as sophisticated as now. But by the third season, the actors say, ‘Well, we have jobs, we’re going to be here awhile, let’s start looking over these scripts.’ That’s when you have to come on strong.”

We’re talking here about acclaimed series, not simply cookie-cutter, mass-market audience favorites. Let’s face it, few who saw an episode of “Who’s the Boss?” in its sixth season likely cared enough to declare, or even notice, “This show has really slipped.”

And it should be noted that those involved with a series can sometimes be too close to it to recognize a slip in quality. Jerry Seinfeld insisted to The Times last year that the show bearing his name was as strong as it had been since it began in 1990, but in a recent issue of Entertainment Weekly, the cast, including Seinfeld, in fact did concede that some of last year’s episodes were on the iffy side. And James Burrows can today say of “Cheers” that he considers its third season to be the weakest, but he didn’t realize it at the time.

Carl Reiner, creator of the classic sitcom “The Dick Van Dyke Show,” which recently was signed for another four years on Nickelodeon’s Nick at Nite, boiled his success down to one thing: “The stories that were interesting were the ones that were possible. The ones with human beings acting as human beings, under stress. So I’d tell my writers, ‘Take the situation and try ‘em on for size.’ If it was possible for me to do something, it was possible for Rob Petrie. That’s what makes a thing last long, if it’s based on true human reactions, forces of behavior that are honest to the human spirit. The reason anything lasts is because the creators are investigating themselves.”

Reiner worked at a time when he had to deliver, as a writer-producer and story editor, 33 episodes a year, not the 22 to 25 of today.

“There were shows we knew we were pasting together, trying to make them work,” he says. “We were aware of those shows, and they were not the happiest to do.”

The decision to close up shop in 1966 after five years, despite high ratings, was an easy one, Reiner says: “We had done five years and knew that if we saw the light at the end of the tunnel, we didn’t want to dog it. We wanted to go out as good as we came in, and we went out a little better.”

To get those human stories, former executive Tinker says, producers must cultivate dedicated talent.

“Good creative people take pride in what they do,” he says. “TV is often considered frivolous or a waste of time, but when it’s good, it’s very good, and when it gets in the hands of good people, they don’t want to look bad.”

One problem nowadays, Tinker says, is that networks are willing to toss money at unproven talent.

“If you’ve written just one episode of, say, ‘Cheers,’ you can get a $4-million commitment from the network,” he says. “That’s too bad; it lowers the level of the work. This happens because there aren’t enough creative people. The next generation needs to be trained over a period of time by established professionals.”

Running dry on creative juices is just a fact of life, Tinker says. The trick is not to do so on the air.

“Larry David is a good example,” he says, referring to the executive producer of “Seinfeld,” who will not return to the show next year after threatening to quit for several years, citing creative exhaustion. “Here’s a guy who’s clearly impatient. Because he contributes so much and is so much a part of the nature of the show, he probably just burned out. He’s anticipating and not wanting to take a dive. ‘Seinfeld’s’ had periods of being more silly than good, but other nights it’s great.”

Larry Gelbart, who adapted “MASH” for TV in 1972 and was one of its producers for the first four years of its 11-season run, had to get out too. He says that while he was on hand in the early ‘70s, the series benefited from a “creative restlessness.”

“We’d try to tie our hands and only tell a story in the Swamp or in a deserted bus,” Gelbart says. “We tried to avoid running jokes as often as possible. We didn’t make it easy for ourselves. It helps to be masochistic--there’s a lot of joy in that if you hit yourself the right way.

“We pioneered the half-hour of multitiered stories. It wasn’t the traditional sitcom style of presenting the situation, then it worsens, then the resolution. We had five, six, seven different stories; some were resolved, some unresolved, some with surprising confluence. We did a lot of that cat’s-cradle plotting. It became like a game, exercises in cleverness.”

But, he quickly adds: “Trying to be clever is taxing. Consider: If I was involved in 97 episodes, if some of those were multi-stories, that’s more than 97 stories, maybe 300. You use ideas up at an alarming rate. I had to get out.”

At that point, the series began to abandon the anarchic spirit of the original film and novel upon which it was based and began taking itself a little more seriously.

“It’s hard to talk about it without sounding disloyal,” Gelbart says of the change in direction. “They did the ‘MASH’ they wanted to. Some were good, some weren’t. But you could say the same about the first four years. I think some of the early stuff is funnier, but when I see some of the early ones, I think I went really crazy, making people too funny too often.”

Gelbart also says that the show was helped by cast shake-ups, as when McLean Stevenson and Wayne Rogers left the series. Indeed, though cast defections or losses are generally greeted with “Can the show survive?” stories from the media, they are in fact a key to keeping programs ever-evolving, says James Burrows, who executive-produced “Cheers” with Glen and Les Charles during its 11-year run, which began in 1982, worked on “The Mary Tyler Moore Show” and devotes his energies these days to directing episodes of such series as “Friends” and “Frasier.”

“An influx of new characters adds a new spice to the stew; it changes the flavor of the stew. When Nicky died, we brought in someone diametrically opposed,” Burrows says, referring to the death of Nicholas Colasanto, who played Coach, and the addition of Woody (Woody Harrelson).

“Instead of being old and stupid, he was young and stupid and made Sam envious of his athletic skills. We didn’t want to do a paternal figure; no one could do that better than Nick. And we were following ‘Family Ties,’ which had like a 51 share. The heat on that show was Michael Fox. So we said, ‘Let’s go younger for Woody.’ That’s how we approached the humor there in a different way.

“And we came up with Frasier to keep Diane in the bar. Lilith was originally a one-shot deal, but Bebe Neuwirth was so wonderful she stayed on. So when they came along, we had two new intelligent characters, which gave it new flavors. When Diane left, it gave Ted [Danson] a new way to play the role. It was about his bar, not about his relationships. The cast changes were incredibly helpful.

“We thought we were out of ideas for a lot of years but always came up with new ones,” Burrows says. “We could have gone another year had Ted not wanted to quit.”

The secret of the success to Lear’s sitcoms, he says, was that “we were paying close attention to the culture. Generally, newspapers gave us a great many ideas for stories. For example, Edith was affected by the feminist movement, and she grew as a result. That meant that Archie had to grow or we had to show why he didn’t grow. Characters grew in the context of the culture. Right now I don’t see a lot of shows dealing with the greater culture.”

Lear also pointed to what can be a double-edged sword for any sitcom--a breakout character with a certain shtick. Jimmie Walker’s J.J. on “Good Times” (1974-79) certainly accounted for much of that show’s popularity, but it also placed certain limits on the writers.

“What becomes over time a little less fresh and is absolutely inescapable is the personality of the actor in each role,” Lear explains. “There’s no way to avoid it. That’s what happened with Jimmie. We never stopped writing about drugs, date rape or social issues. But because of the impact of that character and the way live audiences reacted to it, what you remember was the huge laughs for Jimmie.”

On the other hand, “Family Ties” found a breakout character in the form of proto-yuppie Alex Keaton, played by Michael J. Fox. Show creator Goldberg credits Fox with avoiding becoming a one-note caricature: “He didn’t want to trade on what we had accomplished the previous year. He didn’t want any ‘Dy-no-mite’ thing to become dated.”

Goldberg said that to keep the series lively, “we tried each year to bring on a new writer, a younger writer, preferably one with little experience. They provided a fresh perspective on the show creatively--’Why do you do that, why don’t you do this?’ To see our world through those eyes and see their energy for our characters was helpful.”

He concedes that his show might have run a little longer than he would have liked: “Overall, I felt that around the fifth year, after we won the Emmy [for best comedy writing in 1987], that was a logical end,” he says. “The sixth year was OK; the seventh year, I had half an ear elsewhere. But there was never a day where I sat and thought, ‘I’m phoning it in.’ I never felt that. We got tired, and on some of the beats, I wondered, ‘Haven’t we done this before?’ But no one ever got cynical.”

Then there are the special cases. As the longest-running prime-time animated series, turning out laughs since 1989, “The Simpsons” breaks just about every rule in the book. Groening and his staff can do things that other producers simply physically cannot pull off. (George Bush as a guest star? No problem!)

“The show has had its ups and downs; it comes with the territory,” Groening says. “In our effort to try different things, we don’t always succeed. There’s a temptation, because it is a cartoon, to not heed some of the rules we’ve established, from simple rules of physics to how much merchandise to squeeze out of characters.

“When you work long hours with a room of writers, you can get so punchy that the only thing that wakes you from your stupor is a really wacky joke. Luckily, we work on each episode for six months, and we can make changes up until the last minute. Having that much time is both a luxury and a curse--in many cases, we have to remember the joke was funny the first 20 times we heard it, even if it isn’t on the 21st time.”

“Murphy Brown” is also a rarity: a show that had seemingly lost its way but, now in its eighth season, has regrouped. Star Candice Bergen last year had publicly announced her displeasure with some of the writing and said that this would be the series’ final season, but, pleased with an injection of renewed energy, she agreed to return for a ninth year next fall.

Two of the program’s current executive producers, Rob Bragin and Bill Diamond, were the first who hadn’t worked with the series’ creator, Diane English. Diamond allows that last season was “bumpier than the show is accustomed to--there was a lot of hangover from the baby.”

Indeed, though Murphy’s decision to become a single mom was great for publicity and pundits the country over, it stalled the series’ momentum: Watching the acerbic star reporter cuddle with an infant was, as Diamond put it, “like seeing Mighty Joe Young chained up.” Moving Murphy back to the workplace has contributed to the series’ rebirth.

As with Lear’s ‘70s sitcoms, Bragin notes, “one luxury we have is that we’re a topical show, and we can keep the show fresh by addressing things going on in the political arena and social arena. We count on a couple of things a year that will generate stories for us. We did something on the ’60 Minutes’-tobacco company thing, and we’re looking forward to next season, an election year.”

“Home Improvement,” now in its fifth season, is one of a very few current family-oriented programs to have enjoyed critical as well as popular success. Being a family show is doubly difficult, says Bruce Ferber, one of the executive producers.

“If you write a show with a specific purpose of sending out a message, it’s not going to be a good show,” he says. “But we do embrace the fact that this is a show about something. It’s hip now to do shows that don’t mean anything, and we like the idea we can [have messages]. So many shows are about nothing and proud of it, and it makes us proud to be going the other way.”

“Home Improvement’s” Finestra and Shoenman had previously worked together on “The Bill Cosby Show” (1984-92), the last highly respected family sitcom. It, ultimately, was widely thought to have tried to stick it out for too long; both men had exited the series seasons before its conclusion.

With many sitcoms headlined by former stand-up comics with limited acting experience, a show can grow along with its star’s thespian prowess.

“When we sat down to do the show, we were thinking about [Tim Allen’s] point of view, and Tim at the time was not as strong an actor as he is now, and we had to write shows toward him,” co-creator Finestra recalls. “Now, he can do almost anything. As he’s become a better actor, the writing has exploited that.”

But executive producer Shoenman is pragmatic--you can’t belt one out of the park on every at-bat.

“Our goal at the beginning of each season is to come up with 10 phenomenal episodes, which is hard to hit, and 10 pretty good episodes, and then five which are so-so,” he says. “If we can manage that, I’d be thrilled.”

Carl Reiner’s last piece of advice is deceptively simple: Just don’t think about it.

“Nobody aims for longevity,” he maintains. “You aim to entertain, and if you entertain well enough, it’ll last. Fred Astaire wasn’t thinking about the history books. He was just interested in doing some good dancing.”