(MONEY Magazine) – Here's how my brother Eric spent his Christmas vacation--or part of it anyway. On Dec. 26, he left his home in Manhattan and drove to Newark International Airport, where he boarded a Continental flight for Washington, D.C. just after noon. He arrived on schedule. Deplaned. Had a Coke in the airline lounge. And made his return trip. By 4:30 p.m., he was back in his apartment.

Is this crazy? Not necessarily. Eric made this excursion--the shortest, cheapest one possible--because he needed to rack up 750 frequent-flier miles before the end of the year to reach the Gold level of Continental's OnePass program. He will now get unlimited free first-class upgrades, free companion upgrades and a 125% bonus on miles earned.

My brother--I know--will use every last one of those miles he went to such great lengths to earn. What's really crazy is the huge number of people who don't take full advantage of these programs.

Randy Petersen, the editor of Colorado Springs-based InsideFlyer magazine, says that 75% of all frequent-flier miles go unused each year. According to his calculations, United Airlines alone had 120 billion earned miles scheduled to expire on Dec. 31. American Airlines had another 35 billion to 50 billion. In dollar terms, the forgone miles from those two airlines alone represent more than $2 billion.

Why are people so cavalier about these rewards? Perhaps they've overdosed on loyalty programs that are becoming ubiquitous. "There's the frequent buyer at the yogurt store, the frequent buyer at the Safeway, even my dry cleaner," Petersen says. "It's no wonder we feel it's no big deal if we lose a little." The average person, he says, belongs to eight to 10 programs. (An exaggeration, I thought--until I checked my wallet and found eight cards.)

The big problem is that this is a snooze-you-lose situation. Not only do points expire, but they can also lose value just sitting in your account. Last year, 5,000 AmEx Membership Rewards points earned you a $100 gift certificate at Saks Fifth Avenue, among other stores. This year, they return $50.

If you're going to play this game, I figure, you might as well win. Here's how:

--Aim high. If you're close to hitting the mileage needed to qualify for an elite program like Continental's Gold, make the effort. Just the 125% bonus on miles means you earn free trips in less than half the time.

--Redeem wisely. Don't blow a lot of miles to book a flight that would be inexpensive anyway. Rather than save $200 on a round trip, you might do better if you transfer the miles into a hotel program that will get you two or three free nights.

--Get help. If you've got thousands of miles that you'd like to use but not enough time to wait on hold for a customer service rep to help you book flights--it took me 27 minutes recently to get someone at AmEx to pick up--you can hire a travel agent to do the administrative work. Petersen's plan, Mileage Managers (800-333-5937), costs $49 to $99 a year depending on how much you use it.

Finally, if you're never going to earn enough miles for a free flight, you have two options. First, you can redeem them for magazine subscriptions--at about 2,000 miles a pop. Or you can give the miles to charity. It won't get you a tax deduction, but you'll do a good deed--and you'll free your wallet of at least one loyalty card.