There are a few ways that I find flights. However, how I book my trip depends on if it's for a scheduled trip or if I am looking to fly somewhere spontaneously.   

Scheduled trips

I ALWAYS track my flights using Google Flights. Once I find 1-3 flights that I like, I turn on a notification to track it. Google will then send me alerts if and when the price fluctuates. Most people will say Hopper however, I find Google Flights is the most updated. 

Spontaneous trips

Now, this is the best way to see the world and purchase a cheap flight. When you have flexible dates, you are more likely to catch a deal on a mistake airfare. Some of my favorite sites are

• Going (previously Scott's Cheap Flights) - make sure you sign up for their emails!
• Skyscanner - put in the filter to go Everywhere and they will list locations in order of cheapest to most expensive from your home airport.
• Momondo - similar to Skyscanner.