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As with many stocks, the value of frequent-flyer miles has been falling, and experts are offering timely advice.

Tim Winship, FrequentFlier.com publisher, notes that the relative value of a mile is down from its longtime rate of 2 cents to about 1.2 cents, based on what a frequent flyer would have paid for a free ticket in dollars. With a prediction of 41 million fewer passengers on domestic airlines in 2009, there may be more frequent-flyer seats available. Winship suggests cashing in miles.

WebFlyer.com chairman Randy Peter-sen points out that if you hold on to frequent-flyer miles until fares rise again, the miles retain their value. But with higher fares, he adds, free seats can disappear. Petersen's advice: Use miles when you need to, and look for deals that make it easier to get a free seat.

To make your own decision, do the math. Large U.S. airlines typically charge at least 25,000 miles for a round-trip domestic flight. At 1.2 cents per mile, that's $300. So if a flight costs less than $300, don't "spend" 25,000 miles. If it costs much more, go ahead.

The real challenge might lie in getting a free seat in the first place. WebFlyer's Award/Upgrade Index tracks travelers' redemption success rates by airline and month. At press time, Midwest Airlines (69%) led the major carriers.