Try a flexible fare search

If you’re at all flexible, you can sometimes save hundreds by adjusting your travel dates, often by just a day or two. Travelocity used to have perhaps the best flexible date search option in the industry because it searches 330 days ahead; we say  "used to be best" because it no longer includes international flights (of course, it’s limited, as all many online agencies are, by its exclusion of Southwest, JetBlue, and several small domestic  carriers such as Allegiant Air). To use this feature, simply click on the “Flexible dates” button just below the “to” and “from” boxes on the flight search engine (check these step by step instructions if you’re unsure how this works).

Cheapair.com  goes out 330 days too but it charges $10 per booking vs. Travelocity's $5.

Orbitz has a limited flexible date function that searches a span of only 30 days (click on “Flexible dates”), and here's something interesting: although they claim only to search US and Canadian fares, they in fact also search international routes.  Another very useful tool is ITA Software, which is the technology that Orbitz runs on. You can't book on ITA, but you may find their flexible search easier to use. A big advantage that ITA/Orbitz has over Travelocity is that if they say there are seats available at a given fare on a certain route, they're almost always right, whereas it's more hit and miss on Travelocity. Plus, Orbitz/ITA show international (and all fares) including all taxes and fees upfront, whereas you have to go to the final booking step in order to see taxes on Travelocity. As good as this combo is, they still don't list fares on Southwest or JetBlue (although they may include JetBlue at some point).

Expedia does flexible search only between a few dozen major US cities (click on “My travel dates are flexible”). Sidestep.com and the other major sites have no flexible search capability. Increasingly, individual airline sites are improving their flexible date search functions. American, AerLingus, Air New Zealand, Spirit, and USA3000 all have decent ones and Southwest has a good one.