Faster than nearly anyone thought possible just a few months ago, Americans have jumped back into the market in a bid to rebound from the drubbing they took last year. And while good stock picking and smart bond selection are always important, savvy investors say one basic investment tenet is more vital than ever: Keep costs low.

When the stock market was logging double-digit gains year after year, most investors didn't think twice about how much their mutual fund company charged them-and hardly made a peep if those expenses rose. But with investors still smarting from last year's crash, even with the recent rebound, and experts cautioning not to expect a return to the old highs any time soon, the hunt for lower fees has become a crucial part of a sound investment...