I always go early in the day, better parking, fewer people at the beginning. Being there when they open, gets you a head start on school groups and most group tours.
Go on-line at NASM's web site to get an idea of the layout and where various a/c are before you actually go there and plan your visit accordingly. There are so many different a/c types from such different eras, it is almost impossible not to find interesting a/c that you did not know were there without doing a little homework.
Also check the NASM web site for any special events or curator talks (about specific a/c or displays).
This is a very enjoyable museum with a really interesting collection on display. I enjoy every visit and always see something "new".
If you want pictures of a/c on the floor, that's where I head first to get the best possible shots before too many other people get in the way. Then I head up on the elevated walkways for overhead shots of a/c on the floor and the a/c hanging above the floor.
Don't miss the engine displays
under the walkway going down to the floor level on the right after you enter the main hangar.
If you have time and patience, look at the many artifact cases throughout the museum, especially the displays in the Space Hangar. I generally head for these once the museum fills up with people in the way of my photos.
The traffic getting to/from U-V can be challenging (to say the least), so allow plenty of time to get there (and to return to wherever you started at the beginning of the day).
The volunteer docents are very kind and some are really quite knowledgable. Don't be afraid to engage them in conversation.
Don't bring anything into the museum that you don't absolutely need to have as everything you bring must be checked by security and that can slow down your entry.
They have a decent cafeteria, so you won't have to leave the museum for lunch. If you do leave the museum for a meal, you have a bit of a ride to a restaurant.
Enjoy your visit.