I have experimented a little. A belt sander is not (repeat, not) a good idea. If you have a little sticker that you tried to take off and there was a lot of gummy adhesive left, don't try nailpolish remover. I did, and it didn't get the adhesive off, but the magnetic strip started to dissolve. If the card is still in good shape and pretty smooth, the best solution for removing stuff is elbow grease and a clean rag. Unless somebody has a better idea, and I'd love to hear it.

Speaking of getting stuff off a card, I would like to hear from other collectors about stickers -- do you ever try to take them off, or do you leave them on? The "senior citizen" stickers are tempting to get off, if you have a butch that are all with the stickers.