The more websites you check, the better your odds of finding a good price. Some sites check ticket prices at a number of online travel agencies so that you only have to enter your desired itinerary once. (See Booking Buddy, in Resources below.) In addition to checking the major handlers, also check with discount airlines; comparison engines often do not include discount airlines such as Southwest, RyanAir, Virgin and jetBlue.
When you book with a comparison engine like Priceline, Travelocity or Expedia, it will often add a booking fee (usually $5 per ticket). To avoid fees, use the comparison engines to find the cheapest fare, and if it is through a single airline, go to the airline's website and book directly. If you plan to book a hotel, the comparison engines often have better prices when bundling airfare and hotel, which will override booking fees. Some also have more hotel options available to those who bundle, which can result in additional savings.
Always book at least 21 days out, but from there it has to do with supply and demand and there is no firm rule. Farecast (see Resources below) keeps records of price trends and uses historical data to predict prices; it will advise whether you should purchase now or wait. If your fare goes down after you've booked it, call the airline and ask if it will offer you the price difference. Orbitz offers "Price Assurance"--if another customer books the same flight later at a lower price, you'll automatically receive the difference.
The more flexible you can be, the better your odds of finding a cheap flight. If you are willing to have an extra layover, take a red eye, or fly a longer route, there will be less demand for those flights, which will decrease the price. Some flexibility as to dates can also be helpful; if you're flying around the holidays, try to fly on the holiday itself rather than the day before or after. You can also save if you are willing to fly to or from an alternate airport. Farecast, Travelocity and Orbitz offer this choice in their expanded options.
Some airlines will sell you a ticket at a reduced rate so that you'll book with them, but make up the difference by levying unexpectedly high fees. For instance, if you exceed Continental's baggage allowance, you'll pay $150 extra per bag. Before you book, determine what services you'll need (like checking bags, pillow/blanket, meal service, change/cancellation), and factor this into the price before booking.
