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Have a truly bon voyage knowing you got the best ticket price.

Because airfares change frequently every day, when you book your ticket is a huge factor in how much you'll end up paying. The variance can be $300 or more on a domestic round trip. So how can you tell when you should buy and when you should hold off? Two data-mining websites can help you calculate the odds that you're getting a good price. looks at historical prices for trips on 77,000 city pairs in the U.S. and Canada. Type in your flights, and up comes a "Fare Trend" chart that shows whether prices have been increasing, decreasing or staying steady. A four-star rating system shows how good a particular fare is compared with past prices on the date and route you've highlighted. By considering volatility, you can determine whether it makes sense to wait for a dip before booking. You can't make reservations at, but you can click though to airline websites., one of PC World's 20 Most Innovative Products of 2007, uses roughly 100 variables to determine expected price changes for flights from more than 75 cities. Plug in your details, and you'll see four graphics: "fare indication," an arrow symbolizing rising, dropping or flat prices; "confidence," a percentage based on the site's track record for predictions on the flight you chose; "average fare change," a volatility measure; and a "buying tip," suggesting you either buy your ticket then or wait for a possible fare dip.

Also, consider using the site's Fare Guard feature. For $9.95, you can protect a fare for one week and receive daily e-mail updates to track the lowest fare. If the price drops after you book, you get a check for the difference.

Julie Moline is a freelance writer, editor and editorial consultant in New York City.

This story appears in the August 2007 issue of Entrepreneur. Subscribe »