Here are some pointers about when you can travel for the best price. When to actually book the ticket for the cheapest flight can be found here.
1. Try to travel midweek. Tuesday and Wednesday are typically the cheapest travel days.
2. Travel the off-season. BUT it's the off season for a reason. Make sure you aren't vacationing right in the middle of the wet season with 40 inch rainfall total norms.
3. Try to stay a Saturday night. If you have to travel on the weekend, staying the Saturday night often means cheaper flights.
4. AVOID SCHOOL HOLIDAYS. Labor Day is a horrible time to travel because it's the last 3 day weekend of the Summer.
5.Look for trends. Know your location. If you are travelling to Vegas for a non-weekend visit. It is often cheaper to fly TO VEGAS on Sunday than FROM Vegas. Watch for planes they are trying to fill.
6. Book flights during the workday. Lots of people try to avoid missing workdays and using up holiday time so they take later flights out. Taking a mid day flight is often cheaper.
I realize that booking cheap tickets is an art form, but it is extremely rewarding!