Always test vital parts that could stop the game from being operatable, like flippers , shooter, startbutton, coin acceptors etc.
I clean the games and glasses every time i come around them and wax them about once a month, at least high traffic locations.
Check lamps and rubbers on the fly and make sure you have them with you or keep spares in the machine (i have complete service kits in every machine i operate and has come in handy quite often)