Be Flexible
This works on both ends of the trip. If you can be flexible with when and where you fly into, you can usually grab some pretty amazing deals. Living in Houston gives me access to a few international airports within a 4-hour driving distance so if there's a really amazing deal that can only be had from Dallas' airport, weigh out if the drive is actually worth it or not. The increased options can net some really low prices. The same is true for where you're going. Right now it's cheaper for me to fly into Manchester non-stop from Houston and take a train to London. It might not always be like that, but flexibility means I can make it happen for a much lower cost.
Some search engines let you look up One Way, Round Trip or Multiple Cities so play around with some different options and see what you can find. On my first trip to the U.K. in 2007, it was cheaper for me to fly into Birmingham (BHX) and then fly out of London as opposed to just going roundtrip from a single destination. Same for last summer, I went into Paris (CDG) and out of Amsterdam (AMS) and saved at least $400 on a non-stop flight.
Don't be afraid to explore a new destination and you might save some money!
Compare, Research, Compare
I love Google Flights. It searches a ton of airlines fast and usually comes up with the lowest prices for me. The ease of use keeps me coming back, but it doesn't always give me the best deal. To compare, check out sites like BookingBuddy or Priceline before clicking buy to make sure that you're getting the best bang for your buck.
There's also fare sites like my favorite, Escape Houston, that list super cheap deals and notify you via Twitter or email when new deals become available. They currently check flights from 5 metro areas in the U.S. (Houston, Austin/San Antonio, Dallas/Fort Worth, New York/Newark and Chicago) and have dug up some amazing sales, glitch fares and more. Make sure to click through on their links when booking flights as that's how they stay in business.
Lastly, check individual airlines for sales. Southwest Airlines has some great domestic U.S. fare sales that they don't list on search engine sites. The only way to find them is by digging a little bit on southwest.com. Many other airlines are the same way so if you have a preferred flying choice, check out their corporate site before booking on Google or Priceline.
Got any other tips for booking cheap airfare? Leave them in the comments below!