People in DC love to complain about tourists (as Joel Housman wonderfully illustrates on How to use Metro and How to use an escalator). The District seems to be the number one destination for “family’s first vacation.” I don’t know why you’d take a bunch of babies and toddlers to museums and monuments, but a lot of people do. And tourists don’t think twice about traveling during rush hour or standing in a big group in the middle of the sidewalk. The worst times of year are summer, spring break and during the Cherry Blossom Festival. You’ll want to stay away from museums during these times of year–at least on the weekends. Fall is a popular time for European visitors, but you usually won’t even notice them because, amazingly, they often blend in better than tourists from the States.
Tip: If you’re heading to the Metro and you see a huge mass of people wearing matching T-shirts also heading to the Metro … run. Seriously, run to get ahead of them, otherwise you’re going to be stuck behind them on the escalator, at the turnstiles and on the platform. And if you’re low on patience, avoid the Smithsonian and National Archives Metro stations at all cost.