I seem to recall jiggling the plug on Space Wars, which would give you free time... usually only a few seconds or so, but I remember once getting 8 minutes on the clock that way!
I had a friend who would hang out near Sprint 2 machines, w hich would play 1 or 2 players on a single quarter... when a single player put his quarter in, he would leap forward and hit the 2 player start button and help himself to a game...