One of my best trips there was when it snowed several inches and they shut the freeways down so we took the back roads. I don't think any of the above posters have mentioned Arlington National Cemetery - it was awesome in the snow. And it sure cut down the crowds.
You didn't say what time of year you want to go or if you have kids, but we always try to do major places like Washington, Rushmore, and other national parks in early spring or late fall when the kids are back in school. It cuts down on the traffic, the crowds, and makes parking easier.
Bill