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You've heard it before: The most important thing is to invest in a 401(k) or other retirement account, and if you can't afford to contribute $18,000 to your 401(k) (the 2016 limit), at least set aside enough to maximize the amount your company will match. The challenge is to pick the right funds. "Choose your employer's most aggressive 401(k) fund," says John Claghorn, at RBC Dain Rauscher. "The biggest mistake people make is being too cautious, especially when they're young."