Depending on how badly the back of the disk has gotten, we generally recommend using a 3M scotchbrite pad with some acetone of lacquer thinner. Clean off all the rust and and dirt that has accumulated on the magnet using the scotchbrite pad and the lacquer thinner to break it down. Wipe everything down with a cloth or paper towels until the magnet side looks clean and new.
Then take a knife blade or X-acto blade and go around the magnet and dig out any foreign debris that has gotten embedded into the magnet (this can be ground glass, metal filings, etc. etc.).