For starters, I'd go with Airtran from Atlanta. There are three airports in DC, Dulles (IAD), National (DCA) and Baltimore (BWI). The cheapest is almost always BWI, because Southwest flies there, and the competition keeps the prices low. Try to pair ATL with BWI using Airtran. Otherwise, try to pair Birmingham with BWI using Southwest. Oh, and don't forget about Richmond, VA as an alternate airport for Washington DC. It's only an hour's drive to DC. I think Jet Blue flies into Richmond?
Sidestep.com is also a good resource for booking tickets and seeing all the carriers' prices in one matrix. Southwest, of course, you have to price separately using their own website once you know other carriers' prices.
Good luck!