I copied this from my website (
www.jetlagrnr.com/tips.php?sid=29)
First, hit a multi-airline travel search engine (such as Kayak.com, Expedia.com, or Travelocity.com) to determine the current fares for the flight you want. If you can, experiment with the dates--how does flying out on Wednesday instead of Thursday or Friday change the price? Most sites have a "flexible dates" option that automatically shows airfare for dates close to your chosen range. What about airports? If you're flying domestically, some cities (e.g., London, Paris, New York, San Francisco, Chicago) have at least two airports you can fly into, which can save you upwards of $100.
Second, once you've gotten an idea of what it's gonna cost you on the major airlines, check out low-fare competitors such as Southwest, JetBlue, and Easyjet and see if they can do better. These airlines are generally not included in searches and are oftentimes cheaper, though you'll have to forego meals, assigned seating, and/or other "luxuries."
Finally, visit the site of the airline with the cheapest fare and get the price there. (You never know--that airline might be having a sale.) Now you're ready to buy your ticket. Unless the travel consolidator is unmatched, buy it directly from the airline to avoid transaction fees.
Bonus Step: A note if you have a frequent flyer account with a specific airline: If their fare is, say, $100 more than the cheapest fare, depending on the number of miles you'll be earning from that flight, it may be worthwhile to shell out that extra cash and simultaneously invest in a free ticket to Europe. Do the math.
BTW, I totally agree with Hugh Jass about Priceline flights.