Use 'Em or Lose 'Em

"Whether the expiration period is 12, 24 or 36 months, there's no excuse for losing miles," Winship said. "Only the grossest inattention on the part of the program member should lead to miles expiring."

The opportunity to extend the life of mileage points with activity (earning points or redeeming an award), US Airways' Wunder said, is more consumer-friendly than the programs at budget airlines such as Southwest and JetBlue. Points in budget carriers' programs often expire after a year or two and typically cannot be extended by program activity.

Southwest's Rapid Rewards program grants one point per flight (defined as any one-way trip from an origin to a destination, so no extra points for multiple stops). If you earn 16 points in a two-year period, you get a free round-trip ticket (good on Southwest or its award-program partner, ATA Airlines) that must be used within a year. If you don't reach this level, your points expire two years after they were earned.

Deborah Benton, Southwest's director of loyalty marketing, said points and awards expire because the airline rewards its most frequent fliers. And because members who don't fly much can lose their points, more award seats are available for frequent travelers. (But not as many as there once were: Until last February, award certificates were good for any seat on any flight.)

In addition, spending $1,200 on the Rapid Rewards Visa card yields one Rapid Rewards point, though these expenditures do not extend the life of Southwest's points or awards. You can, however, pay the airline $50 to extend the award for another year.

AirTran Airways' program is similar to Southwest's. Each one-way AirTran trip is worth one point in its A+ program; 16 points are required for a free round-trip flight. Points expire after a year, and awards, issued automatically when sufficient points accrue, are voided a year after they're issued. An award can be extended by a year, AirTran spokesman Tad Hutcheson said, if the A+ member requests a voucher before the award expires.

JetBlue's TrueBlue program automatically credits awards when enough points are earned. If these awards aren't used within a year, they're lost. In addition, TrueBlue points expire after a year unless extended by using the JetBlue American Express card. The twist: Each time you book a JetBlue flight or spend $200 with the card, you earn a TrueBlue point -- and whenever you earn a point in this way you extend the life of all your points by a year, said JetBlue spokeswoman Alison Eshelman.

Given the continued decline in value of frequent-flier points, the best strategy is to use miles for award tickets or upgrades as soon as feasible. Because not only do mileage points not earn interest, if you bank them for too long, they can disappear.

Michael Shapiro is the author of "A Sense of Place: Great Travel Writers Talk About Their Craft, Lives, and Inspiration" (Travelers' Tales).