Just got back from a DC trip about a month ago.
I only read through a few posts, but definately get tickets online for everything you can. National Archive reservations are online and maybe $1-$2each and is SO worth it. The line is huge and we got to walk right in. Long lines on the left, reservation door to the right of the building. They even stopped the security line and let us go through. It was great.
Holocaust tickets are online as well and I HIGHLY recommend going there. You will not regret it.
White House tickets we were able to get. I wrote our state senator 6 months in advance. We got the confirmation about a week prior. Stood in line for over and hour and a half to get in. It is extremely limited, but there is just something neat about saying you were inside and walking where so many great people have walked. A few interesting pieces as well. Go to the White House Tour office place before going. They offer a video that kind of gives you a tour before the tour. It was very helpful and we were glad we did it before hand.
All the smithsonians are free and are well worth it! They are really great. The little lunch area at the Museum of History ,I believe it was the history one, is a little costly but the salad bar rocks!

They have lots of yummy options there.
The metro is super friendly. If you have an Iphone or something with Apps I got a Metro app that was SOOO worth it. It allows you to put in where you are and were you want to go and tells you which Metro to take, how much it costs, etc. They offer a day pass, but make sure to note you cant ride until a certain time when using it. Oh and do not eat or drink while waiting on the metro. We almost got fined.
I know they say parking is hopeless, but unless you are staying at a hotel near the Metro (we were not. were actually in a condo in williamsburg and drove to dc 4 or 5 days. will never do again. ha!

), we spent a fortune paying to park and then ride the metro. It was convenient but got costly. If you read the signs near the National Mall, parking is allowed like 930-4 ... If you plan to get there at that time. 9 to 915ish it is not hard to get a free parking spot. It was great for the Washington Monument, Smithsonians, and National Archives. Just remember to note the parking times! We actually got towed. Haha. Luckily we only had to walk a couple blocks to get the car and we paid $100 online when we got home for the fee.
Arlington Cemetary is also well worth it! We took the metro out there. I cannot remember how much the little trolley tours are there, but they are worth it. You still do a little walking, but not near as much if you just went on your own. The guides also have some great tidbits.
We did a moonlight trolley tour which was $30ish or so a piece. You meet at union station and its a couple hours long. I am glad we did it because it was a nice way to see a lot of monoments, but we did not go to every monument they said we were and the guide was super annoying. We had time limits at each stop, but once everyone was back on the trolley the guide would talk for 15 minutes. A couple tidbits, great, but 15 minutes of chatter while we are at a dead stop? Id rather be spending more time at the monument... definately check out some monument tours because the ease of pulling right up to each one was great. Seeing all the monuments up close is just awesome. Definately get out at night because what they say about everything looking better at night is SOOO true!
I have a few guide books and maps. (some got a little wet, but are still readable) If you want to PM me your address I would not mind sending them to you. Im sure I will never need them again.
Thats all I can think of right now.... oh! There are street vendor carts everywhere. We stopped at one near the White House/Smithsonian area for a giant egg roll and it was seriously the best egg roll ive ever had. Haha. That was some home made cabbage goodness! It was like $3 and I wish I had one right now.
Oh! Speaking of DC food I remembered another. There is a little cafeish type place near the Navy memorial (not far from National Archives) They have pizza, sandwiches, hot and cold bar. The food was greatly prices and SOOOO good. I cant remember the name, but if you are standing at the entrance to the Navy Museum it is directly to your left on the corner. Cant miss it. Yum!