A cheaper, although a bit less frequent way is to take bus B5 up to Rosslyn Metro station. Or if you are going to somewhere near L'Enfant Plaza anyway, take it all the way up there.
Schedule at
http://www.wmata.com/timetables/dc/5a.pdf
Originally Posted by GUWonder
http://toandfrom.org/airport/va.html
Well, a combination with public transport (Washington Flyer bus combined with the metro) is the best way to not deal with the variations in traffic on DC-area streets/highways.