The cameras should be required to be on in order for the officer to do anything "in the line of duty". Furthermore, they should broadcast a wifi feed of what they're seeing in real time so that anyone can have a "listener" running nearby to capture it. You could have a listener in your car, at your apartment, or running on your iPhone. There should be a listener in the police car that instantly uploads the feed to a server managed by an agency that is totally independent from the police department.

Camera always on. Everyone gets an immediate copy.