(MONEY Magazine) -- When you've got your plan in gear, you really can relax. All you need to do is minor maintenance one day a year. Pick any day for this checkup: a week after your birthday, the day after New Year's, whatever.

Start by revisiting your asset mix. If market downturns have been giving you sleepless nights, you may have taken on too much risk. Consider shifting to a more conservative asset mix by moving money from stock funds to bond funds.

Playing it safer is something you should do anyway as you get closer to retirement; you can't afford to be caught by a market slump that near the finish line.

If you have a do-it-yourself portfolio rather than a target-date fund, this checkup date is the time to rebalance. In other words, it's time to correct for the fact that during the year, gains in some funds and losses in others throw your asset mix out of whack.

Compare your current allocations with your targets. If they are off by, say, 10% or more, transfer enough money out of your winners into your losers to get back to your original mix.

By doing this every year, you force yourself to sell high and buy low - the very definition of smart investing. In most plans, you can rebalance with one phone call to your 401(k) provider or with an online tool.

And, of course, if you hold a target-date fund, you're off the hook: All this is done for you.

Ignoring your 401(k) for 11 months and 29 days every year may seem like slacking off, but it's far, far better than tracking your returns too closely. Obsessing over performance could entice you into the classic mistake of chasing yesterday's winners.

Benefits consultant Hewitt Associates found that in 2005 many 401(k) investors loaded up on emerging markets funds, which had been delivering double-digit returns. But in May of this year, foreign markets tanked, and panicked investors found themselves selling with 20% losses.

"When you chase the highest-returning funds, you often end up selling at the bottom," says Lori Lucas, Hewitt's director of participant research. That's why you want to pick an asset mix (or target fund) that you can stick with through market gyrations.

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