So you buy an airfare at a ridiculous price -- or even a good one. And a week later, the airline decides it hasn't sold enough seats. Rather than fly an empty plane, which would cost it money, it slashes the prices on seats. Come the day of the flight, you turn to the person sitting next to you and learn, to your disgust, that they paid $200 less than you did.
What can you do? Well, by the time you're on the flight, often nothing. But if you discover you've paid more than you had to before you have used the ticket, you can usually petition the airline for the price difference. Usually, that refund comes in the form of a voucher that you use for future travel, but that's still money you don't have to spend later on.
But, surprise! Some airlines have a nasty trick up their sleeve. Many charge obscene change fees since, the way they see it, they have to pull your old ticket and issue a new one to give you the better price. That means that for domestic flights on U.S. Airways, Continental, and American, the price has to drop by more than $150 in order to give you an ultimate benefit. But plenty of other airlines don't charge any fee at all (JetBlue, Alaska, United), or their fee is small enough to give you pretty good chances (Northwest's is $50, AirTran's $75). The fees are usually steeper for international flights, but then again, the price drops stand to be higher for those, too.
After you book a flight, you could keep returning to the airlines' websites to double-check the rate status of your booked flights. That will work. But one of the lesser-known airline booking sites, Yapta, keeps tracking the price of the stuff you've bought, and if it descends past the point where you can actually make some money back, it alerts you by e-mail. (The site, like Hotwire's Trip Watcher and Farecast, will also keep an eye on rates for flights you haven't bought.) Every bit helps, right? Maybe the amount you save will pay for a pack of peanuts. Barely.