If you are going to choose an actively managed mutual fund rather than an index fund, fees may be the most important factor, experts say.

"The first and most important rule to picking a mutual fund: stick to funds that cost the least," writes Wall Street Journal columnist Joe Light.

Active managers generally underperform index funds because their fees are higher, not because they lack skill in picking strong stocks and bonds, Matthew Morey, a finance professor at Pace University who has written multiple studies on picking active managers, tells Light.

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"If anybody asked me how to choose, and said just to pick one factor, I would say 'Just buy low fees,'" Morey explains.

For some asset classes, including municipal and high-yield bonds, there are even actively managed funds that have lower fees than index funds.

You also have to be careful to choose a fund that truly is actively managed, Light writes.

Martijn Cremers, a finance professor at the University of Notre Dame, tells Light that active managers are increasingly structuring their portfolios to resemble their benchmark indexes, a strategy known as "closet indexing."

Certified financial planner Dee Lee offers several tips for selecting mutual funds on the CBS Boston website. First, check for a fund manager with a strong record for at least three and five years, preferably 10 years, she says.

Second, seek a fund with an average or lower-than-average expense ratio. Third, make sure that turnover isn't excessive. And fourth, look for low volatility, Lee says.

Editor’s Note: 250% Gains Bagged Using Secret Calendar (See Video)

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