Not tricks to get free games, but tricks when playing the games
I used to find old joust machines and after the 4th wave teh 'lave hand' could hold down the enemy in place. if you had an old Joust machine the pteradactyl would fly up and you could ALWAYS joust him in the mouth just by standing on the middle ledge. I got so good I could sit there for hours just bouncing back and forth between pteradactyls.
I wish MAME had a setting that allowed you to still do this in Joust. I haven't seen it work in the version I have...