Try the aggregating sites Expedia, Orbitz and Yahoo. Plus the airline sites, because they are not all included in the other sites. Southwest, I think, is not included by the aggregators.
Monday and Friday flights are generally most expensive.
Evenings are usually cheaper (NOT Friday and Sunday evening, though).
Don’t buy way early or way late (unless you come upon a great sale). When I flew a lot, I found that prices dipped about 4 or 5 weeks before the flight.
Sign up for the airlines’ weekly email. Most of them send out deals on Wednesdays.