Glad to have one more person commuting in the metro DC area.

Personally, I wear my work clothing (slacks, jacket, and tie) and just treat the bike as a car -- hop on, and go. This works for me because

a) my trip is only 10-12 miles round trip,
b) my bike is an upright european city bike with fenders, chainguard, lights, etc., and (probably most significantly)
c) I'm not in training for anything, so my level of effort is about equal to walking.

I'm not sure how relevant my experience is to you, as I have a far shorter trip and don't ride very hard. Unless your company provides you with a reserved parking space in Old Town, the motivation to ride, rather than drive, should be easy to find.

Paul