Having lived there for most of two years (just a few blocks from the White House and National Mall) here’s some advice:
1. Don’t go during most of the summer (late June, July, and all of August), the city is sweltering and the only people who stay in the 110 degree heat and 50% humidity are those who do not know better.
2. If you’re going to play around the mall and the Smithsonians, most are open 10am-5pm, every day, and morning is better as most of the school trips full of screaming adolescents haven’t unloaded yet.
3. Do not buy a pink, baby blue, or any other pastel colored FBI, CIA, or Secret Service item of clothing. No, they are not cute.
4. The FDR memorial is one of the trickiest to find, but easily the best of all of them!
5. Wear comfortable shoes. Unless you intend to spend an enourmous amount of money on cabs, you will be walking a lot! Sneakers are a must have!!!
6. Don’t take cabs unless neccessary. This is not NYC, they will rip you off if they even smell tourist on you. Heck, I still get ripped off by them.
7. STAND ON THE RIGHT, WALK ON THE LEFT!!! This is VERY important for all escalators! If you mistakenly stand on the left, you will automatically tick off everyone and get sent waves of bad kharma from all the commuters who are already grumpy from spending half an hour on the Metro to get home.
8. Speaking of the Metro, do not call it the subway. You can “metro” it to places or take a metro, but you cannot subway it unless you want a sandwich.
Okay, that’s all I can come up w/ off hand. :-D