At first glance, travel booking sites that search multiple airlines can look pretty much the same; while they do often vary in the specific features they offer, particularly in flexible-search options, they really are a lot alike. This may lead one to assume they offer the same prices on the same flights. This is a mistake.
Each of these sites has its own deal with the airlines they search, so prices can vary [source: Airfare Watchdog]. Your best bet is to search them all -- or at least three or four.
Know the sites' policies, too: While many travel sites have eliminated booking fees, some do still charge, typically about 10 bucks. (While you're at it, you may also want to check out the airlines' policies for bag-check fees, carry-on fees, seat-selection fees, change fees and charges for onboard snacks.)
Once you've searched the booking sites, make another stop before you buy.