(Following is an updated version of an article that first ran on Sept. 21, 2001.)

NEW YORK (CNN/Money) - Like a thin person who's got serious mojo for metabolism, your 401(k) never seems to gain any weight, no matter how much you feed it.

With the major indexes down steeply this year -- following two straight annual declines -- it gets harder to feel good about pouring hundreds of dollars into your 401(k) every month.

So, you may be thinking: Why bother? But this is no time to give up. Nobody knows when the downturn will end, but you'll want to be there when it does. "The trick is staying in the game. If you opt out, you lose," said certified financial planner Mari Adam of Adam Financial Associates in Boca Raton, Fla.

There's nothing like a few numbers to illustrate the point, especially on days when stocks are swathed in red and investor confidence is taking another body blow.

To demonstrate the penalties of missing market upturns, Doug Flynn, a certified financial planner with FlynnZito.com in Garden City, N.Y. cites a study by The Hartford Financial Services Group. According to the study, you would have averaged a 13.9 percent annual return if you had been invested all year, every year in an S&P 500 portfolio between Jan. 1, 1970 and Dec. 31, 2000, a period during which there were nine bear markets and 10 bull markets.

If you had missed the best 10 days of the market during that period -- the days when the market booked the most gains -- your average return drops to 12 percent. The more "best days" you miss, the lower your return. Had you missed the best 60 days (just two months out of a 31 year period), you would have earned a mere 6.3 percent a year.

AVERAGE 401(k) BALANCE       • 2000: $40,918 • 2001: $36,390 Source: Cerulli Associates. Average portfolio composed of 66% broadly diversified stocks and 34% bonds.

If dollars and cents speak to you more directly, consider the following example from the Hartford study. Two investors start out the 1973-'74 bear market with $100,000 invested in 6-month CDs. During that bear market, the Dow lost more than 45 percent.

One of the investors decides to keep the $100,000 in CDs until Dec. 31, 2001, ending up with a nest egg of nearly $832,000. The other investor, however, moves 50 percent of her money into S&P 500 equities on Sept. 30, 1974, a month before the bear market ends. By Dec. 31, 2001, her nest egg grows to more than $3 million.

The only reason not to contribute to your retirement plan now is if you don't have an emergency fund worth three-to-six months of living expenses. "Access to cash is paramount," Adam said.

If your emergency money is in place, the priority should be to continue saving for your future, especially your retirement. Besides getting a tax deduction and possibly a company match for your 401(k) contributions, you're also maintaining a savings habit, which like a muscle weakens without use.

"Once the engine slows down, it's real hard to start it up again," said certified financial planner Barbra Steinmetz of Steinmetz Financial Planning in Burlingame, Calif. "Money is like closet space. The more you have, the more you use."

And even though your 401(k) balance looks less like the Green Giant than the grass he tramples on, you're getting more for your money. "Investing in troubled times is investing in opportunistic times," said certified financial planner Joseph Janiczek of Janiczek & Co. in Greenwood Village, Colo.

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Put another way, "We're looking at 1998 prices on the market, and it won't stay there forever," Flynn said. "If you've got 20-plus years to retirement, you should be sinking every dollar you can on a monthly basis."

Indeed, Adam noted, "This is dollar-cost averaging at its best. Instead of buying 50 shares as you did last year, you're getting 100 today. When you retire in 20 years, it'll be worth a lot more."

Of course, just throwing money at a 401(k) account is not the smartest strategy in any kind of market, good or bad. It all depends how you invest it.

Planners recommend you stick with your asset allocation if it's appropriate to your age and goals. (Our asset allocation tool can help you figure out if you have the right mix of stocks, bonds and cash.)

But if it's off the mark, "bite the bullet" and fix it, Flynn said. It's never easy to realize losses in funds that have done poorly or to pull out of funds that are doing well, he noted. But good allocation has to be your top priority since it's one of the best protections in down markets and offers you the best chance to benefit from upswings in different sectors. "No one really knows what will be the best (asset) class," he explained.

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If your 401(k) plan doesn't offer you enough funds to satisfy your allocation needs or has a type of fund you need but which is sub-par -- that is to say, it underperforms its peers and has an expense ratio that would make Gordon Gekko blush -- invest in the best choices your plan offers and make up the difference in your IRA, where you can invest in top funds that complete your allocation pie, Steinmetz said.

And if it still makes you queasy throwing as much money as you have into equities, you might temporarily redirect some of your new contributions to a more secure type of investment, such as a bond fund, a balanced fund, a stable value fund or a real estate fund. "A fear of further losses wouldn't stop me from contributing to a 401(k). It's just where I would put it," Steinmetz said.

Like it or not, your 401(k) is a big piece of your retirement puzzle. So as tempting as it may be to put a hold on future contributions until the world turns right-side up, it could damage your prospects in the long term. "The markets alone are not going to give you the retirement you want unless you continue to feed that pot," Steinmetz said. "You can't afford not to."