Recently a couple friends asked me how they could report trash, graffiti, potholes, and such. The best way I know is through the Mayor's Citywide Call Center: either dial 311, or go to 311.dc.gov.
I've used it especially for gang tags, and they usually get painted over or removed pretty soon. However, the Call Center usually only applies to public property (sidewalks, alleys, streets, street signs, and the like). The data also helps the city determine where it needs to focus and where to do Fix-Its, where they walk around and fix stuff, so it's worth it to report issues you see.