Use Skyscanner to book the cheapest flight possible. Skyscanner is my favorite flight search engine as you can find flights around the globe and from every airline possible. You'll be in great shape to get the best price possible.
You must grab a free trial or join Scott's Cheap Flights Premium to get cheap flight alerts directly to your email inbox. My favorite cheap flight alert was 25,000 Delta SkyMiles to London non-stop.