Washington Flyer Coach leaves IAD at :15 and :45 after every hour. It leaves West Falls Church Metro station at the same times. The ride between the two takes about 20 minutes.

As pointed out in previous posts the choices are:
Washington Flyer Coach (costs $9 one-way) - then two subway rides costing a bit more than the 5A combination.
5A Metrobus (costs $3 one-way) - leaves every 40 minutes M-F and every 60 minutes Sat,Sun,Holidays - then a single, shorter subway ride.

Notes: The Washington Flyer Coach leaves IAD from Arrivals Level Door 4 (UP the ramp from baggage claim area at door 4) - buy your ticket just inside door 4.
5A Metrobus leaves IAD from Curb 2E (DOWN the ramp from baggage claim area at door 4) - exact fare required on the bus.

Additional Note on 5A bus: Yesterday, today and tomorrow (July 9-11) are all Code Orange Air Quality days (check status at http://www.mwcog.org/environment/air/forecast/). On Code Orange or Code Red days the 5A Metrobus, like virtually all public bus service in Northern Virginia, is FREE.