If you have a really dirty coin that looks horrible, then use soap and water perhaps. If it still looks awful, then keep it until you can find a better example. If you polish it, it'll be hairlined and worth a fraction of whatever you paid for it. (Which is pretty silly, unless of course all of your coins will be bought for 20p each) Collectors go through many runs of upgrading before they feel comfortable with a particular coin, that's what you need to do. Don't ruin them for future generations, polished coins look awful. They look artificial, they get hairlined and they never get back to how they were. If you want one that looks nice, save up, don't buy rubbish and buy coins that have remained shiny looking from new. You pay a premium for a BU coin because it's escaped being ruined by a bloody idiot in the past. Far be it from me to tell you what to do with any coins you may own, but please, don't be a pleb!