1. Look at historical data. By examining pricing history, a number of relatively new sites tell you if fares are heading up or down, or alert you to unusually low fares. At http://www.farecast.com, for example, do a search for one of the fares for the 75 domestic cities tracked by Farecast, and the site not only makes a prediction but indicates how sure it is of that prediction. Farecast also has a product called Fare Guard. For $9.95, it will lock in a price for one week, but only for fares it predicts will stay stable or go down during that time. Other sites that offer fare trending include http://www.farecompare.com and http://www.kayak.com, which recently merged with SideStep.com (both maintain a Web presence).
2.Check with an aggregator. Booking aggregators -- including http://www.sidestep.com, http://www.kayak.com and http://www.cheapflights.com-- scan numerous booking sites and cull the results. Most also display hotels and car rentals.
Aggregators work in one of two ways: Either you download a program or go to a Web site. SideStep works both ways. You can go directly to its site and plug in your dates and other information. In the downloaded version, the SideStep toolbar pops up and runs a concurrent search when you're on another site; when it's done, it directs you to where you can book.
3. Go to an all-purpose travel site. The big three ( http://www.orbitz.com, http://www.expedia.com and http://www.travelocity.com) remain dominant, because you can book your flight, hotel and car in one spot. All have exclusive Web deals, but some airlines aren't represented, fares can vary wildly and the sites charge a booking fee of $5 to $15.
4. Sign up for instant alerts. Web sites such as http://www.travelocity.com, http://www.kayak.com and http://www.orbitz.com offer fare-alert services that you can customize. Type in your parameters, and you'll receive an e-mail when a fare dips below or matches the price you've set. Likewise, download such features as Southwest Airlines' Ding ( http://www.southwest.com), Orbitz's Insider Deals and Expedia's Fare Alert, which provide instant notification on your computer of special fares.
5. Look at airline and airport sites. Go to airline sites to see whether they can match the lowest fare you've found. You often can get extra frequent-flier miles for booking directly with the carrier, and you'll avoid the service fees on some of the all-purpose and aggregator sites (as well as the fees charged by some airlines if you call their reservations numbers).
Note that some discount carriers are not widely represented among the discounters and aggregators, including Skybus ( http://www.skybus.com), which offers a number of $10 seats on all of its flights; Philly and Richmond are as close as it gets to Washington. Fares for Virgin America ( http://www.virginamerica.com), a discount carrier that started service from Dulles last year, can be found primarily on its Web site and on Orbitz.com.
Also, check flight schedules on the airport sites -- http://www.bwiairport.com and http://www.mwaa.com-- to make sure you're not missing a carrier that flies a specific route but might not show up on the major booking sites. Southwest, for example, doesn't sell through third-party providers. USA 3000 ( http://www.usa3000.com), which flies to Melbourne, Fla., and destinations in the Caribbean, is another airline you must check directly.
Sign up to receive the airline e-deals that surface each week (usually midweek), and go to http://www.washingtonpost.com/travel and subscribe to the Travel section's weekly newsletter, which includes the latest airline e-fares. For late-breaking deals, including those with a short shelf life, go to the Travel section's blog at http://blog.washingtonpost.com/travellog.
6. Check Priceline and Hotwire. Though http://www.priceline.com and http://www.hotwire.com provide regular flight-booking services (showing airlines and flight times along with prices), both offer potentially money-saving twists. On Priceline, you can bid for a flight, then discover the airline and times after you pay; check http://www.biddingfortravel.com for bidding pointers. Hotwire shows you the price upfront, then reveals the carrier and flight times after you've provided your credit card number.
7. Check other budget sites. A number of sites -- including http://www.cheaptickets.com, http://www.travelzoo.com, http://www.smartertravel.com and http://www.independenttraveler.com-- are frequently updated and full of bargains. The Web site for Budget Travel magazine ( http://www.budgettravel.com) has tons of tips, as well as deals and a frequently updated blog.
Each Wednesday, Travelzoo unveils its Top 20 deals, which include airfare specials, packages and hotel bargains. Look on the site or sign up to have them e-mailed to you.
8. Consider last-minute specialists or auction sites. Check out the packages at services such as Lastminute.com ( http://www.lastminute.com), which offers late-breaking air-and-hotel combos. While you might not need the hotel, the package price could very well beat the no-advance-purchase fares being offered elsewhere. Or go to a site such as http://www.skyauction.com, where you can bid for an airline ticket; just remember that once you buy, you're stuck with the ticket.
9. Turn off the computer . . . and contact a travel agent. Many charge fees for booking a ticket, but a good agent will know where to look for cheaper fares and can give you pointers on how to find them. Also, for complicated itineraries, such as an around-the-world ticket, check with an agent who works with consolidators; many consolidators don't deal directly with the public.
Check for an agent's good standing with the American Society of Travel Agents ( http://www.travelsense.org).