Yeah, using dirty shots is a great way. Have the camera look through a window, shoot from under a table (as Mel Brooks satirized in High Anxiety), shoot from behind a plant or the edge of a doorway. Stuff like that. Also, blocking your actors so they sometimes have their back to the camera (because obviously they don't know it's there) could add to the effect, just don't overdo it, you don't want the whole film to just be peoples' backs.