An old iron, from Goodwill (about 3/4 dollars), works. Lay the keys on the bench and place the iron (on a medium setting) directly on top of the keys. You can cover about 8 to 9 at a time. About 30 seconds later the key tops pop off. Sometimes need a little persuasion with a putty knife.
This PTG archive page provided courtesy of Moy Piano Service, LLC