I've seen some good things done with putting silver into a bath of dilute lemon juice (or any ionic solution, like salt water) and aluminum foil. From what I understand of the chemistry, it strips off the oxygen atoms from the tarnished silver and deposits it onto the aluminum. It supposedly is a near perfect reversal of the silver tarnishing process, in a way that can't be done with structurally-damaging volume-expanding oxides like rust. Do some research yourself - I'm no expert - but it might be worth a try for buttons or if you can remove the cantle from the sporran.