Scan current offers on deal aggregators. Airfare Watchdog, TravelZoo and Smarter Travel staff scour airlines' websites and sales to cull the most recent deals. Look for current offers for your destination and time frame.
Search with a booking site. Kayak pulls real-time fares from airlines and allows you to narrow or widen your search to certain times of day or to regional airports to find the best deal.
Look at fare trends. Kayak's charts will show historic prices for your route, allowing you to see if fares are increasing or decreasing. For U.S. domestic flights, Bing Travel will use a complicated analysis to predict if fares will increase or decrease.
Check regional and low-cost airlines. Southwest Airline's fares do not appear on booking sites, so you must check their site. Flights on European, low-cost and regional carriers are included on Momondo. Cleartrip aggregates the smaller Asian and Southeast Asian carriers.
Set fare alerts. If you have not found a flight through the above methods, booking sites will send a daily e-mail with fare prices or an alert when the fare decreases. Yapta allows you to set an alert for a specific flight and time instead of a general route and day.