One of the most important decisions you will make for your retirement plan is how to divide your investments among stock funds, bond funds and short-term investments such as money market or stable value funds. Your choice should be guided by your risk tolerance and the amount of time you have before you plan to retire.

"Using the target-date funds that a lot of 401(k)s have in them now is a good kind of cheat sheet," Lane says. "You look at the target-date funds for your retirement age, pull up a chart on that fund and look at the volatility. Then you decide whether or not you're willing to take that rough of a ride. If you're not, then back down to a more conservative target-date fund."

Even if you're not investing in a target-date fund, you can follow its asset allocation formula as you design your own portfolio. Beth McHugh, vice president of market insights for Fidelity Investments in Boston, says that as long as you're within 10 percent to 15 percent of the mark for your age group in each investment category, you're generally in good shape.