Know the times
There are timings in which you can be admitted as well as a time when you must exit the locality. Know these times and also do some research for the times that are most busy and least busy. This will help you in picking a day and time where you can enjoy the museums more and do not feel rushed.
Know the route
Washington D.C. is known for its notorious parking problems and taking a car is quite hard in the extremely busy traffic. The parking is not only hard to find and is extremely expensive so you better make use of mass transport.
You can catch a subway that will drop you off very close to the institute or you can opt for a metro bus as well depending on where you are coming from. Taking a cab is another option if you wish to skip the crowd on public transport though it will cost a fair bit more.
Get there on time
Make sure that you reach the institute as per your planned time so that you can enjoy it thoroughly. This will get you started in the right direction.
Know what you want to see
There are eleven museums on the National Mall so you should plan ahead of time as to what you want to see and what you want to skip. Make sure that you know where you interest lies and give it the top priority in planning.
If you have more than a single day, it is not a bad idea to dedicate one day to only one or two museums. This will allow you to have a closer look and be able to study the museum better. This is possible if you are a resident of Washington D.C. or its suburbs.