MANAGING YOUR MONEY / Earning More, Keeping More : NEST EGG SCRAMBLE : <i> Most Americans haven’t put a nickel toward retirement, but it’s vital to begin saving today if you want to live comfortably tomorrow.</i>

A startling survey by Wall Street giant Merrill Lynch recently found that more than half of all Americans haven’t saved a nickel for their retirement. And the survey was only the latest in a growing body of evidence that indicates the gilding is falling from the “golden years.”

Indeed, other surveys say that roughly eight out of 10 Americans--some 76 million households--will have less than half the income they need to retire comfortably.

With the over-50 set the fastest-growing segment of the population and the Social Security system on shaky ground, it’s vital to begin building a nest egg today, experts say.

One of the easiest and most effective strategies to provide for retirement might be right under your nose. Most larger companies and even many smaller ones offer their workers 401(k) plans, which allow you to save nearly $9,000 annually for retirement. These company-sponsored, tax-favored accounts are generally considered the best option for retirement savings.

But if you don’t have access to a 401(k), it’s worth seriously considering opening an individual retirement account, or IRA.

What’s so special about 401(k) plans? They are low-cost, high-return and easy.

These plans are offered directly through employers, so you can make contributions automatically with a weekly, twice-monthly or monthly payroll deduction.

Many employers will also match a portion of your contributions. In other words, they’ll pay you to save. And contributions are pretax, which means Uncle Sam is contributing part of your savings. You don’t have to pay him back--or pay tax on your investment earnings--until you pull the money out at retirement.

To illustrate how all these factors work in concert to make 401(k) plans one of the best retirement savings vehicles around, consider two hypothetical taxpayers.

* John Kay, 20, earns $25,000 annually and contributes 20% of his gross pay to a 401(k) plan. That’s $96 a week.

Kay’s employer provides a 25% match, which means the company will contribute $25 for every $100 he puts into the account. In this case, the company puts in $24 a week. Kay’s retirement plan grows by $120 each week, or about $6,250 a year, before figuring investment returns.

But because 401(k) contributions are taken out pretax, Kay’s paychecks haven’t been reduced by the full $96. The net difference in his checks, in fact, amounts to just $69 per week, or $3,600 annually. In a backhanded way, Uncle Sam has kicked in the other 28%, or roughly $27 weekly.

Let’s say Kay continues to contribute the same amount for the next 20 years and earns an average return of 9% on his investments. He ends up with $349,457 at the end of two decades, even though his out-of-pocket contributions were just $72,000.

The benefit of 401(k) contributions is greater for those in higher tax brackets and for those who save for longer periods.

* Consider Sally George, whose gross pay is $75,000 annually and who is taxed at a rate of 31%.

George pays in nearly $173 a week. (Her pretax contributions are limited to $8,994 annually in 1993.) But because the contributions are made before taxes, her weekly paycheck is reduced by about $119.

Assuming her employer also matches 25% of her contributions ($43.25), George’s retirement account will grow by $11,245 annually before investment earnings. Assuming a 9% annual return, George’s account is worth $629,750 in 20 years. Her out-of-pocket cost: $123,760.

If George continues to save at the same rate and investment return for an additional 20 years--a total of 40 years--she’ll have contributed $247,520 but she’ll have accumulated $4.4 million thanks to the power of tax-free compounding.

Additionally, 401(k) plans are more flexible than most other retirement savings options. Although the plans vary a bit from employer to employer, many allow you to borrow against the savings in your account--usually at low interest rates--without incurring tax penalties.

There are usually some limitations on 401(k) borrowing, however. For instance, many plans allow you to borrow in case of a medical emergency, to pay for college or to buy a new home. But they might not allow you to borrow against the account to buy a new car.

You might also be allowed to borrow only a percentage of your account’s total value--perhaps 50%. While that’s more restrictive than a normal savings or investment account, that’s very flexible for a retirement account.

“401(k)s are freebies,” says Gregg Ritchie, partner in KPMG Peat Marwick’s personal financial planning group in Los Angeles. “There are other ways to save for retirement, but there’s none that’s better.”

What if your employer doesn’t offer a 401(k) plan? Consider an IRA.

The benefit of an IRA is that anyone can contribute and defer tax on the investment earnings in their IRA account. The detriment is that contributions to IRAs--the amount you actually invest each year--are only tax-deductible if you meet certain guidelines. If your company offers a pension plan and you earn more than $25,000, for example, your IRA contributions will not be fully tax-deductible.

But if your company has no pension plan or you earn less than that, you can deduct up to $2,000 in annual IRA contributions. You just have to note the IRA contribution when you file your tax return.

There’s one caveat with IRAs. While you can direct how the money is invested, you can’t spend it without paying a hefty 10% tax penalty, unless you are older than 59 1/2.

If you’re younger and wanted to use the money to buy a house, for example, you’d end up paying nearly 50% of it in federal, state and excise taxes regardless of whether you borrowed against the IRA account or you withdrew it. For that reason, pre-retirement IRA withdrawals should be used only as a last resort.

Although you would also have to pay the penalty if you withdrew the money from a 401(k), you can usually borrow against the account and avoid the penalty altogether--one more reason they’re better than IRAs.