• The New York Times, Tuesday, April 1, 2008 11:15 AM

Airline frequent flier programs--which began 27 years ago as a way to win the loyalty of travelers--have turned into a lucrative business for the airlines. Many now earn hundreds of millions a year by selling miles to partners like credit card companies and hotel chains. Those companies, in turn, give the miles to customers as sign-up bonuses or rewards for hotel stays.

But as consumers can earn miles in more and more ways, the number of miles accumulated each year has vastly outpaced the number redeemed, an imbalance many airlines are now being forced to address. One way that carriers are trying to reduce the mileage backlog is by adopting stricter expiration policies. Miles now typically expire after 18 months or two years of account inactivity, instead of three years.

Airlines are also offering more ways for members to use their miles, like bidding for things like Broadway tickets at online auctions, trading miles for merchandise or using a combination of cash and miles for air travel.

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