My husband was in a clinical trial at NIH in March and April of this year. As a member of the HiltonHonors loyalty program, I stayed most often at the Hilton Garden Inn or the Doubletree, both in downtown Bethesda (the HGI is a little nicer and better located). I also signed up for a HH American Express card, which gave me 30,000 bonus points, enough for a free night. I checked in and out of the two hotels, depending on which was the cheapest, and was able to work the program so that I earned a free night for about every 4-5 paid nights (my husband also had points, so that helped). I found that, generally, the weekends were pretty cheap ($89-$135), and the weeknights were pretty expensive (often $250+), so I always used the points for a weeknight stay. The farther away from NIH you go, the cheaper rooms get, but I liked being only a few minutes away. The first four nights of the trial, I stayed in a basement apartment rented through vrbo.com (
http://www.vrbo.com/228268). The owners were very nice, and it was relatively inexpensive, but I just felt more comfortable in hotels.
Good luck.