M.C.
Kayak.com will show you the current fares available on many sites all at once--you can use this to see which is cheapest, then go there to book. I agree that booking directly with the airlines is usually best--and cheapest--but you can save yourself the work of going to each site individually by going straight to kayak first. They don't do southwest, however--you have to check them separately.
You can also go to farecaster.com to get a 30-day trend on airfares, to see if a certain route is likely to up or down. Unfortunately, for most routes, the past week or two have been higher than avg--for travel next month, I'm not sure if you'll want to wait and see if they go down, or not.
For the future, you can also subscribe to email deals from hotwire and travelzoo, so you'll get news of the best sales available each week. In addition, you can sign up for kayak to track a particular route for you--i get an email whenever rt dallas-boston is under $300. GL!