(MONEY Magazine) -- For years Frank Valone never had a problem cashing in frequent-flier miles when he traveled between Syracuse and his home in San Francisco, even at peak times. Then, out of the blue sky, airlines denied two consecutive requests to redeem miles for domestic tickets this summer.

Valone, 25, ended up shelling out $800 for one of the flights, over Memorial Day. The trip earned him 6,000 more miles, but so what?

"It has gotten so much worse recently," he says. "They give you all these miles but then it's impossible to redeem them."

Trouble with redeeming miles has been building for several years.

Sure, the largest U.S. airlines increased the number of award seats issued last year by 6.5 percemt, according to the consulting firm IdeaWorks.

But airlines have also taken to handing out miles like candy, through credit-card purchases, hotels and car rentals, causing a nearly 14 percent surge in the number of unused miles out there.

Result: That increase in seats doesn't mean much. And getting a seat is only getting harder, says Tim Winship, publisher of FrequentFlier.com. In the fourth quarter of 2005, the percentage of travelers flying with frequent-flier tickets hit its lowest point since 2001, according to Back Aviation Solutions. Winship says the current travel season is the worst in history.

It's unlikely that the miserly airline industry will alleviate matters by turning many revenue-generating seats over to frequent fliers, considering that today's flights are about 80 percent full, their highest level since 1945.

So if you've got a lot of miles, chances are you have very few flights to choose from.

How can you land a seat?

Click, then talk Start with the Internet - booking online saves you the $10 to $15 that airlines typically charge to make reservations over the phone.

But if you don't find what you're looking for there, you'll increase your odds by 30 percent when you pick up the phone, says Randy Petersen, editor of InsideFlyer.com.

Creative phone agents may be able to find you an itinerary that accommodates both your travel schedule and your unused miles by searching connecting flights and alternative hubs.

They can also check partner airlines for available flights, which most Web sites can't do.

Go for a combo The big airlines offer two levels of award tickets: domestic restricted seats, which generally require 25,000 miles, or unrestricted seats, which cost 50,000 miles.

"Out of desperation, an increasing number of consumers are paying more miles," says Winship. "They are effectively being forced into upgrading to an unrestricted award."

Before you blow twice as many miles, ask if both directions of your flight are full at the restricted level. Some airlines allow you to purchase one leg of the trip for half of the restricted level, or 12,500 miles, and the second direction for only half of the unrestricted 50,000 award, which totals 37,500, still less than the unrestricted seat.

Also, domestic first-class seats generally start at only 40,000 miles, so ask about flying in style, even if only one way.

Time it right Ideally you could book your trip the very day an airline put the flight on sale, generally 330 days before departure. (Also watch for airlines to announce new flights.)

But because that requires serious foresight, and because airlines don't always make every frequent-flier seat available that day, Petersen recommends booking a more realistic six months in advance.

For the start of summer, that means planning in December, just ahead of the busy spurt in January. Try to leave midweek rather than on a weekend.

If you're taking a spontaneous trip, check airline Web sites to see which destinations are open to frequent fliers. You just might discover someplace new.

_____________________________

Last-minute travel bargains

Steal your travel agent's tricks

Cheat your way to elite status