I get asked almost daily where I find all my cheap flights! Anybody who knows me knows I've gotten some crazy deals since I started exploring ($99 LA to Iceland, $94 LA to Stockholm, $320 RT from SF to Hong Kong etc). 2 things factor into these finds! 1. My schedule is extremely flexible, if I see a cheap flight I grab it and work around it. 2. I check EVERY. SINGLE. DAY.
My two main sources are theflightdeal.com, I check this everyday to see if anything magic popped up on the radar, the other is Skyscanner - they have a super convenient feature that lets you say I'm traveling from "Wherever you are" and I want to go "everywhere." What makes it even better is there is a "cheapest month" option so you can really find some gems!
Happy hunting fellow travelers!