Well, you could try googling Fare Specials, sometimes they change day to day. You might also look at third party sites like Kayak and Hotwire..keeping in mind that Southwest flights will not be on them.

Some airlines allow you to "re-fare"...meaning if you pay x on tuesday but on thursday the price is y, you can have it adjusted/partially refunded. The best time to book is 6 to 4 weeks out. Closer in, they generally up the prices because they have a better sense of the demand for the flight. Farther out, they get what they can to see what the market will bear.