(MONEY Magazine) – A year ago, Bryan Noyd, a Wenatchee, Wash. restaurateur, left the cold, wet Northwest for a warm, sunny week in Cancún. He and his wife and kids flew first class and stayed in roomy quarters at the Hilton Cancún, where the three- and nine-year-old loved the pool and beach. The best thing about the vacation, Noyd says, was the much needed family time. Second best: Noyd paid for the entire trip with frequent-flier miles and hotel points.

That's no easy feat these days because your frequent-flier miles aren't what they used to be. For starters, with more people vying for a limited number of free seats, it's tougher than ever to land one. The onslaught of discount carriers means the value of a free coach seat has dropped from about $500 to $300. And new fees for redeeming miles by phone or in person instead of online—$15 at United, for example—can make that free ticket not so free.

Ordinarily, all that would be just a hassle. But the financial woes plaguing the airlines poses a real risk: If you don't use your miles soon, you may lose them, says Tim Winship, editor of FrequentFlier.com. To bring the miles you've accumulated to a smooth landing, you need to know three things.

WHO'S IN TROUBLE Among the big six traditional carriers, American, Continental and Northwest are fairly healthy. Delta and United are in trouble but are not in imminent danger of shutting down. (United filed for Chapter 11 bankruptcy protection in 2002, and Delta may soon, but that doesn't affect frequent-flier programs.) US Airways is in the worst shape. Already in Chapter 11 for the second time, it has, in Winship's estimation, a relatively high probability of going out of business.

In the past, when an airline stopped flying, a surviving carrier adopted members of its mileage program—American picked up TWA's routes and pilots, for example, as well as its frequent-flier program. But today, no airline is in strong enough financial shape to do that, says InsideFlyer.com editor Randy Petersen.

WHICH MILES ARE SAFE TO EARN You can keep racking up American, Continental and Northwest miles—by flying or by charging on the airline credit card—without a worry. With Delta or United, try to hedge your bets. That's not to say you should stop flying these carriers if they have the most convenient or affordable flights. But you may want to switch your spending to American Express or Diners Club cards, which give you points that can be shifted to a number of frequent-flier programs (including Delta's). When you fly US Airways, you have the option to earn United miles instead. Take it. And if you have been hoarding US Airways miles, book a trip now. If the airline goes under, other carriers may honor your ticket (although none have to), but miles sitting in your account will be worthless. Better yet, use US Airways miles to buy a ticket on United or another member of the Star Alliance network of airlines.

HOW TO SCORE A SEAT You can do what Bryan Noyd does: Book 331 days in advance, when the airlines load their new inventory of award seats into the computer. It's a strategy that's taken him to Mexico, London and Paris. If you can't plan far ahead, book late. Airlines start releasing empty seats to the mileage programs two to three weeks before the flight. Another strategy is to be flexible. Ask (as I have done) where you can go that's likely to be warm in mid-February. As a last resort (I've done this too), cough up twice the miles, which will give you many more seat choices. When you're soaking up the sun or schussing down the slopes, I guarantee you won't regret it.