Mylar for high wear areas & metal protectors for holes. Mylar on the shooter lane. PETG washers for the plastics, or game-specific plastics protectors. Mylar around the flipper buttons to prevent paint cabinet paint wear. Metal cabinet protectors for the legs.
Clean, wax, and replace balls frequently, depending on the amount of play.
Be prepared to check on the game frequently. Often times, problems will go unreported.
Check the coin mechs regularly for jams. Install a bill acceptor.
Keep a selection of rubbers on-hand, especially flipper rubbers. Keep a couple coil stops on-hand, as well as a flipper rebuild kit. Also consider a flipper coil and common playfield coil. A machine that's down won't be earning, so you'll want to minimize that as much as possible with a selection of some common replacement parts.