4. Use flexible date searches.
Adjusting your dates of travel by just a day or two can save hundreds, especially if you're buying for more than one person. This is one area where online travel agencies such as Orbitz, Cheapair, Cheaptickets, Hotwire, and Travelocity shine and often have an advantage over most airline sites and meta search engines. Travelocity will show you the lowest published fare (irrespective of seat availability) over a 330 day period on all domestic fares (except those on Southwest, Allegiant and a few niche carriers) and on major international routes; Orbitz and Cheaptickets do the same over a 30 day period of your choosing (on nearly all domestic and international routes, and they do a better job of assuring seat availability than Travelocity does). Look for the "flexible travel" box or "Find low fares for flexible trips" link on these sites and follow the instructions. In 2009, we predict that more airlines will offer some kind of flexible date search tool; last year, several added or improved this useful device.
See tip 5.