One of the world's great mysteries is when exactly the best time is to buy airplane tickets. You want to wait just long enough so that the price hits its lowest point --- but you can't wait too long, or else it'll skyrocket again. Is it four weeks out? Four months?

You can never know. Even with the help of tools like Farecast, Kayak and Farecompare, there is always the risk that two days after you purchase your family trip to Europe, the price of your itinerary is going to plummet.

What most people don't know, however, is that many tickets can be repriced. In the same way that you can take a television back to Best Buy if you find out it went on sale the day after and get the difference, you can also exchange tickets weeks after you book them.

Airlines, of course, are a little more criminal stingy about the process. Almost all of them charge a rebooking fee that can vary between $50 - $150 dollars. But on a high priced ticket, that can be a fraction of a price fluctuation.

The key is to keep an eye on your ticket price, even after you purchase it. If you see your exact same itinerary drop significantly in cost, call the airline up and ask to refare or rebook the ticket. They'll dig around to see if it's worth your time after levying the rebook fee and if you're lucky, they'll issue a voucher for the difference in price.

Just last week, for example, I noticed that a huge fare sale to Salt Lake City affected the ticket price for a future itinerary that I have into Reno. Calling up Delta Airlines, I got them to issue me a $54 e-cert for use in future travel. Sure, it's not cash in hand, but I can definitely use the voucher.

What this stragety is particularly good for is repricing business travel. Many travel agents will blindly book a ticket on a preferred airline at high cost. When you, the passenger, check back in and reprice the ticket later, however, the reward is yours.

Filed under: Airlines, Budget Travel