While the above search engines are great, they do not always include small airlines, especially in less popularly booked routes and/or in remote regions. If you're flying somewhere obscure, Google search and ask around if there exists a local airline. While in South America we learnt that the LADE Air in Argentina (run by the military) has cheap flights to Patagonia, which is of course not listed in mass search engines online.
This will give you a ton of options if you are traveling
Check for student discounts. That’s right, student discounts go beyond the movie theater and football games these days. Sites like statravel.com focus on finding flights at a discounted rate for students only. Don’t let the high prices of textbooks and tuition prevent you from travelling any longer; keep up with student discounted flights and head off on your adventures whenever you’d like.
Follow airlines on social media sites. You may feel silly adding an airline to your “liked” pages on Facebook or following them on Twitter, but the benefit of doing so is the special offers you’ll receive. Airlines want to reward regular customers and encourage them to continue flying with them, so they will often advertise special low rates and discounts via social media sites only. Follow as many airlines as you can for the best chances of getting a sweet deal.
In Google Chrome or Safari, incognito is enabled by hitting Command (or “Control” if using PC), Shift, “N”. For Mozilla Firefox or Internet Explorer, hit Command (or “Control” if using a PC), Shift, “P”. This will open a new browser window where your information is not tracked, thus not inflating prices as you search. Note: if you're using an older version of OS X, open Safari then click “Safari” in the menu bar, and select “Private Browsing”.