There is a metro bus from downtown DC near the main sites to IAD. http://www.wmata.com/bus/timetables/view.cfm?line=49
There are also metro trains from smiithsonian metro station (train says towards Huntington) to dca. You can take the blue line towards http://www.wmata.com/rail/

The website is useful as is metro's trip planner. http://www.wmata.com/rider_tools/tripplanner/tripplanner_form_solo.cfm