How to Get the Most Out of Your Frequent Flyer Miles When's the best time to cash in? You've done the hard part: You've racked up thousands of frequent flyer miles through trips, airline malls, and your airline miles credit card. Now you've got to figure out the best way to use your miles. If you're not planning a trip in the near future, you need to make sure to retain the miles you've earned. "Keeping them current is hardly an effort," says Joe Brancatelli, publisher of the travel website JoeSentMe.com. Brancatelli says most airlines require some type of activity in 12 to 18 months, depending on the airline. (Delta miles don't expire, for example, and Air Canada miles expire after seven years, regardless of activity on the account.) Activity doesn't mean you have to redeem miles, however. For example, you could make a purchase with your airline miles credit card or shop at the one of the airlines' online malls. Shopping at online airline malls not only prevents your miles from expiring, it can also earn you bonus miles. For example, some airlines will give you six, seven, or even eight miles per dollar when you make purchases online from stores like Best Buy, Crate & Barrel, and Gap through the airline's online mall, says George Hobica, founder of Airfarewatchdog.com. "Anyone who buys anything online and is not going through the airlines' shopping malls is throwing away easy points," Hobica says. Keep in mind that you'll likely have to