How to Be a Party-Perfect Guest : Follow a Few Simple Rules: <i> Always </i> RSVP, Be Charming--and Know When to Leave

“Don’t hit the person across from you with bits of toast,

And don’t, when dinner is nearly through, say ‘Who’s the host?’

It isn’t done.” --Cole Porter

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Elizabeth Mayhew was hopping mad. Before her last dinner party, many of her guests failed to RSVP. They just assumed she knew they were coming to the party.

“People have become incredibly lazy and insensitive,” says Mayhew, an assistant to the food and entertaining editor at House Beautiful magazine. “They get busy or they forget about it, and they don’t respond to the invitation. It gets to the point where I have to get on the phone and call people up and ask them if they’re coming to my party.”

This sort of confusion happens all too often, turning a pleasant holiday get-together into an exercise in frustration. To sidestep tensions in socializing, we polled etiquette experts on how to be a good guest. Here’s what they said:

* RSVP. It doesn’t matter how you respond--by mail, phone or voice mail--just make sure you do it. “People think to themselves: ‘What’s one more person?’ so they just show up without giving the host or hostess notice beforehand,” Mayhew says. “But there’s nothing worse than running out of food or plates because you’ve miscalculated the number of guests.”

* Before shopping for a gift, check with the hosts. If they say not to bother, respect their wishes. If you want to bring something anyway, but you’re not sure what to get, experts suggest a bottle of wine. “It’s not too expensive or elaborate, and if the hostess doesn’t want to serve it that evening she need not do so,” says Elizabeth L. Post, author of several books on etiquette.

* Nix the armfuls of flowers at the door. It’s better to send flowers earlier in the day, experts say, or after the party, as a thank you for a special evening. “When people bring flowers the night of the party, they’re putting the hostess in a delicate situation,” says Judith Re, author of “Social Savvy” (Fireside). “She probably has her floral arrangement already, and this upsets the design scheme.

* Notify the hosts beforehand of food requirements or restrictions.

* Listen to the hosts. If they say they’d rather you not help serve or clean up instead of helping to serve or clean up, take them at their word.

* Try to be charming and friendly. According to manners mavens, guests have a responsibility to talk to other people, including people they’ve never met. Post says a good guest is “enthusiastic, congenial and considerate.”

* Know when to stay and when to leave. Post says guests should remain at a party at least an hour after dinner, because it’s hardly complimentary to the hosts to “eat and run.” At a small gathering, she says, try not to leave long before everyone else seems ready to go, since your departure may break up the party.