Researchers at T. Rowe Price assessed how shifting from a relatively modest 40 percent stock allocation to a go-for-broke 80 percent stake would affect the prospects of a catch-up saver, someone 55 years old earning $100,000 with $150,000 in savings.

After running thousands of market simulations, they found the larger stock allocation wasn't worth the bumpier ride. The 40 percent stock portfolio replaced only 27 percent of pre-retirement income, while the 80% portfolio replaced an almost identical 28 percent on average.

The reason: Though stocks historically deliver higher average returns over the long run, over any 10-year period you are more likely to endure some losing years, and there is less time for your gains to compound.

You could luck out and enjoy outsize returns, but the odds say you won't. "By the time you reach your fifties, the game is pretty much over," says Christine Fahlund, senior financial planner at T. Rowe Price. "And by loading up on equities, you may end up ratcheting up risk without significantly increasing your nest egg."

Instead of betting the farm on stock, you should create a well-diversified mix of U.S. and foreign stocks, bonds and other assets. One solution: a target-date retirement fund, a fund of funds that automatically shifts allocations to become more conservative as you approach retirement. (Both the Vanguard and T. Rowe Price series of target funds made our Money 70 list of recommended funds.)

Or you can build your own customized mix by using the Asset Allocator.

And to prevent a last-minute retirement disaster, lighten up on company stock in your 401(k). If your employer runs into trouble, you could see your stock crash and lose your job too, as Enron employees did. That's why many financial advisers recommend keeping no more than 5% to 10% of your portfolio in company stock.

Until recently some employers required you to hang onto your shares. But under the Pension Protection Act, companies must now permit 401(k) participants to diversify out of employer-contributed stock, typically over a three-year period. Be sure you do.