You could also take the Washington Flyer bus service (www.washfly.com) from IAD to the West Falls Church metro station. Takes about 30 mins and costs $9 one way. From there you can take the Metro (www.wmata.com) orange & blue lines (transfer at Rosslyn) for $2.75. That part takes about 20 mins and will deliver you right to the terminal at DCA. Might sound a bit complicated but actually very easy to do unless you have tons of luggage.