If time if no obstacle, you can also take the free shuttle from IAD to the Metro station (it's about a 20-minute ride, I believe), take the Metro over to Union Station, and hop the MARC train (if you're going on a weekday) up to BWI rail station for just a couple bucks. Then there's a free shuttle bus from the BWI train station to the airport. I've done it a couple times before. I needed about 90 minutes at rush hour, if I recall correctly. Those were with little waiting time, too.