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Don't you dare open that monthly statement you get about your retirement account, says Jack Bogle, founder of the mutual fund giant Vanguard, which now has about $3 trillion of assets under management. "You're gonna get a statement every month," says Bogle. "Don't open it. Never open it. Don't peek." Wait until you actually get to your retirement, then you can open your statement (although, Bogle jokes, you may want to have a cardiologist on hand). Not knowing how much you have growing in a retirement account makes you less likely to want to raid it when your kids go to college or when you want to buy that shiny new car. It also makes you less likely to trade in and out of the market, which can be a fool's errand.

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