Use waxes, car/motorbike waxes. Apply with cotton, rubbing and buffing it till it shines, but make sure u clean the racket well 1st, esp. the ridges, waves or around the grommet holes. However, this only applies to shiny finishes, for matte finishes u can't do much except cleaning them. I own and managed an automotive refinishing /autobody repair shop.
You don't need a good one, anyone will do. Get any cheap brand that still works, usually sold in tubes to polish motorbikes helmets in Asian countries. Or u can also use toothpaste, autosol (chrome polish). Most important is to rub it using cotton and the compound should be creamy n no particles in it.
