I have a method with a 97% success rate. If I'm at a location (quarters, timed play, doesn't really matter) and I'd like to play a certain game, I'll hang back. If the player credits up again without checking over their shoulder, I'll ask (between plunges, so I'm not interrupting too much) "Hey there. Mind if I play next after this game?" The response is almost always "Oh, uh, sure, ok."
Most of the time it's someone who either isn't up on pinball etiquette or is sorta not paying attention to their surroundings. Once in awhile a person is just being a jerk on purpose, but I never get yelled at/cursed at, etc. Just a friendly request to play later on, after they are done. Many people are just a little clueless at times (I know I am) and this seems to go over better than saying 'its my turn to play right now' or quietly fuming until I vent out my verbal frustration.