Frequent Fliers: Make Sure You Don't Get Clipped

For millions of Americans, frequent-flier miles have become a second currency. In addition to piling them up by hopping on a plane, you can get them by making phone calls, buying toys, investing in mutual funds, taking out a mortgage, or renting cars. "Get 5,000 Bonus Miles FREE," screamed a recent credit-card offer from Banc One to United Airlines' MileagePlus members.

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