Use sites like kayak.com to comparison shop.
Fly at off-peak times if possible.
Be flexible with dates. Be flexible with airlines. Take a connection if you can spare the trouble, the layover time is short (but not too short), and the cost savings are large.
Even be flexible with destination - Nonstop to Cancun might be better a longer flight with connection required, to Hawaii.
Some of the travel sites even have a cool feature that shows you all the places you can go for a certain price or less. Neat to see what $500 will get you.
Fly with carry-on luggage only, if possible - lets you make last minute changes such as to accept a "bump" from you scheduled flight, for which they'll pay you a fairly decent voucher towards future travel.