The best way for an unheralded rookie at the back of the depth chart to make a team is to contribute on special teams. They seldom get on the field for snaps from scrimmage. Teams are reluctant to risk their top level players on special teams for fear of injuries so the lower tier guys get their chance.
Take Nick Bellore for instance. There was nothing remarkable about the undrafted rookie from Central Michigan. His chances of making the roster seemed slim at the start of training camp. A strong preseason with good work on defense and special teams earned him a spot. He was still in peril as one of the last men on the roster. One tempting free agent could have meant the end for him. Then he became a special teams ace, unofficially leading the Jets with over 15 special teams tackles.
This usefulness for the NFL's version of slave wages will keep him on the roster in 2012 and give him a chance to develop as a linebacker he otherwise would have been unlikely to get.