There are all sorts of techniques and theories for getting cheap flights. Buy on Tuesdays. Book travel that leaves mid-week. Look for deals via Twitter. The list can go on and on.
My number one trick for getting good deals is simply to know what the average price is and be able to buy when you see something that is good. If I have an upcoming trip I want to take I set up a price alert on Kayak and I simply check the price for the particular flights I want every day. Over the course of time you get to see a general trend on what the average cost of a ticket is, then if there is a sale or suddenly the price drops I’m able to know it is a good deal and I buy it right then and there!
This is also is handy when booking popular routes like NY-SF or LA. Over the years I’ve observed that the average price is around $400 for a direct return flight. If I see a direct flight for the dates I want that is say $300 then I’ll just buy it right then because I know that is about as good as you are going to get.
So in short, know the average price by looking frequently and you’ll know when you see a good deal!