Earning frequent-flyer miles has become something of a national pastime: You can rack up the credits by flying as well as by buying – everything from contact lenses to condominiums.

But collecting the miles is easy; cashing them in is another story.

As many travelers are discovering, there is a good chance that you’ll have to make annoying changes in your travel itinerary and spend twice the expected points to get the free flights you’re after.

We confirmed as much earlier this year when we examined frequent-flyer programs at 10 of the country’s largest carriers – eight “traditional” programs that award tickets earned with miles, and two “nontraditional” programs whose low-cost airlines reward travelers with points or credits for flights taken.

While searching for 300 coach-class trips to destinations on the mainland and Hawaii, we learned a few tricks that enabled us to snag 93 percent of the flights we requested – albeit with some compromises.

Here’s how you, too, can work the system to get free (or nearly so) seats:

¢ Focus on the best programs. Many airlines are cutting back on award seats. Continental, for example, reduced free flights last year to 5.6 percent of its total inventory from 7.6 percent in 2003. America West trimmed awards to just 1.5 percent of all seats, down from 2.1 percent the year before.

Yet most airlines will guarantee you an unrestricted seat on any flight that’s not sold out – for twice the minimum miles requirement. The best programs, we found, are those that do this the least.

American Airlines’ AAdvantage – our top-rated traditional program overall – wasn’t markedly better than other programs at finding us award seats, but it charged extra points in only 20 percent of our bookings. (Northwest’s WorldPerks and Alaska Airlines’ Mileage Plan also did well in this regard.) By contrast, our lowest-rated traditional program, America West’s FlightFund, charged additional miles 60 percent of the time.

¢ Be flexible. Our testers were able to get flights with the minimum number of miles by agreeing to fly off-peak, mid-week, very early or late in the day, or by using alternative airports. And if you can plan within two weeks of your departure, you may be able to find flights that weren’t available months before, says Randy Petersen, editor of Inside Flyer, a magazine and Web site for frequent flyers.

¢ Online or phone? You make the call. Booking frequent-flyer travel online usually saves a small fee, but in many cases testers who found nothing online later succeeded with a phone representative. In some instances, representatives had access to flight information unavailable online, especially for Hawaii, a prime frequent-flyer destination.

¢ Ask about first and business class. If you can’t get a coach seat for less than 50,000 miles (double the minimum requirement for most destinations in the continental U.S.) you may be able to find room at the front of the plane for the same amount – or less. We did on several flights. Of course, low-fare carriers such as JetBlue and Southwest – proprietors of the two nontraditional programs we rated – don’t have business- or first-class seats. And neither flies to Hawaii.

¢ Keep your account active. (Once you’ve found a good frequent-flyer program, anyway.) Most airlines require that you have some account activity in the previous 36 months to prevent your miles from expiring. (In some cases, all you need do to keep your account active is use your airline credit card.) The exception in our rated traditional programs was Continental, which erases miles after 18 months.

JetBlue’s otherwise excellent TrueBlue program drops you after 12 inactive months, while you’re jettisoned from Southwest’s RapidRewards after 24.