This post actually applies to any wooden furniture your cats aren't allowed on but sneak on while you're sleeping. You can imagine when, several days after buying a new piano, I found a small scratch in it that wasn't there before. The cats had learned not to jump up on the piano, but apparently after we went to sleep each night it was party central. The nice piano tuner buffed out the scratch for me and I tried a variety of gimmicks to keep the cats off the keyboard. (Just for the record, the worst idea I tried was a slipcover. This was more inviting than the piano itself and each morning I found the cats happily napping on it.) Here's my solution:
You know, those rolls of liner that come in a variety of sizes and pretty patterns? Just cut the liner to fit the piano (I just use it on the sloping keyboard area) and put it on whenever you're not playing the instrument. Even if the cats jump up on it there won't be any scratches and the liner won't slip off.