Large online travel agencies like Expedia, Orbitz or Travelocity have discounted or web-only fares. These fares are negotiated between an airline and a travel agency may not be available in other reservations systems. Compare offers from different sites and travel agents before you buy.
Don't forget to subscribe for news from airlines. They will send you information about discounted fares, promotions and sales. Usually they will inform you in advace about up-coming campaigns, so you can prepare yourself for fare-chasing. Check your e-mail frequently and good luck with buying the cheapest airline tickets for your next trip!