Park at Greenbelt Metro Station then fly BWI via new B30 route from Greenbelt station. 
Parking is FREE on weekends and on federal holidays, and during weekdays IF you EXIT the
parking lot before 2:00PM and after 10:00PM, otherwise, the fee is $2.75 (as of 7/2003) regardless of how long have you parked there.
The service is run by WMATA. started Tuesday November 13, 2001.
The buses run every 40 minutes from 6am to 10:50 pm weekdays, and 8:40am to 10pm weekends and holidays. Travel time is only 30 minutes. 

For schedule information (in PDF format), please click here

Further great news on this new bus connection between Greenbelt Metro and BWI Airport: Riding FROM Greenbelt, the fare on the BWI express bus is only $2.50 one way, or, only $1.65 if boarding the
bus with a Metrorail-to-bus transfer. On the way back, you'll pay $2.50 to ride from BWI to Greenbelt. If you've left your car at Greenbelt station, the parking fee is $2.75, pay on exit until 10:00pm weekdays; or if exiting after 10pm weeknights or anytime on weekends, the parking is free!! 

With a deal like this, why would anyone from the DC area who is flying, drive and park
at other airports?