One of the best spots to check for cheap flights (notably excluding SouthWest) is on Google Flights. Generally, you'll find cheaper options booked a month or more out, with options on some carriers being cheapest as soon as they are available to book (i.e. SouthWest).
Enter Departure City/Cities/Airports, & Consider Looking for 1 Way Pricing 1st; The More Cities You Enter, the Lower the Prices You Might Find
Flexible To Field Options
Narrow by Price
Look Out for Google Pop Ups
Explore Using the Date Grid & Price Graph
Flexible Dates
Avoid Phantom Options On the Left with No Pricing
Play with the Map Visually To Expose Hidden Options You Won't Find Otherwise Due to a Persistent Google Maps Glitch
Consider Hiding Separate Tickets (Or Not)
Set Price Alerts
Most images courtesy https://www.google.com/travel/flights