If you get stuck paying a lot for your tickets, keep checking fares and watch for price drops. Sign up at yapta.com which keeps track of prices and airline policies.
Also, try buying tickets at least 21 days earlier to get the best price. Try checking bing.com's airfare predictor which helps you whether you should snap up a ticket or wait.
Sometimes, you can get lowest fares on an airline's website, and sometimes at Kayak or Expedia which locate fares from a combination of carriers.
Check flights that depart on a Tuesday, Wednesday or Saturday when fares are generally at their lowest. Take note as well that on many routes, travelers pay a premium for round trips that don't include a Saturday.
If your trip don't allow a Saturday stay over, consider one-way tickets and try buying 2 one-way tickets from low-cost carriers like Southwest or Jet Blue which don't require round-trip purchases to access the lowest prices.
Check for alternate airports where in some areas, a discount airline may not have any routes to the major airport, so try checking for a smaller one in the area.
Sign-up for emails from your preferred airlines and get access for special deals.
Check farecompare.com which scans airline data and sends alerts for your favorite cities, while airfarewatchdog.com has experts hunting for good deals.