Look at screens like this one:
http://gamesurf.tiscali.it/screenshots/08/unch1229414312108/uncharted-2_437430.jpgThere's a lot that can be done to fill streets or something, try to add ordinary stuff like trash, papers etc. These little details we don't even care about until they are missing. Look around at your hometown's streets how they look, what they're filled with.
