It really depends upon the instrument finish but, generally, nothing.

For poly finishes - I don’t have any of them so use whatever you like! For nitrocellulose, a damp cloth but if that doesn’t work a minimal amount of lemon oil (if I can stand the smell). For French polished finishes “Super Nicko”. And, before you ask, you can get it from Beare & Sons Ltd., Dunstable, LU6 1NE.

If it’s scratches you’re bothered about, try “Nova” plastic polish. It comes in grades 1, 2 and 3. Which is the most abrasive, I’m not sure - I’ve only ever had a bottle of grade 2.