As one who travels to DC a couple of times a year, I'll say that the advice offered to date is spot on.

No one has mentioned socks. This is a minor point to some, but one that gave me great consternation in my early years. I only buy black. This avoids confusion when your in an early morning fog, especially if you've been out late the night before (if you know what I mean). I like padded or reinforced toes and heels. They are more comfortable if you're on your feet all day, and just plain last longer.

Keep everything clean and pressed and you'll do fine.

Dean