If you are going into downtown DC, take the Metro. It will get you
near pretty much anything you might want to see, and parking
downtown is not just difficult, but practically non-existent.
(Though, see next tip if you are going to the memorials.)
The memorials are open later than the museums, and if you wait
until after 5 PM, you can easily park downtown close to the
memorials. This saves a lot of walking.
You might want to consider getting tickets for a DC tour. There
are usually several different tours that you can buy, depending
on the museums and memorials that you want to see. This is
another great way to cut down on the walking.
There are also tour companys that basically offer a shuttle
between DC sites. For example, the Old Town Trolley offers a tour of 17 DC
sites, and allows you to get off and on at any of the stops.