Depends on where in DC you are going to. If it is on the tourist trail, drive to New Carrollton Metro Station (I-95/495 and US 50). Take the Metro to your destination and back. If it is off the beaten path than get some google directions and drive there. Plan accordingly for rush hour traffic. From DC to BWI, I used the Amtrak service to BWI Amtrak Station from Union Station in DC. You can also pick it up at New Carrollton. The Local MARC Commuter train runs on the same routes from the same stations. There is regular shuttle bus service from the BWI train station to the airline terminal.
