Basically, do not clean coins, it should be outlawed! Although, obviously if you have a coin that is caked in muck, get rid of the muck as best you can, by hand or by running it under a tap. Roman coins and other coins that have been in the earth for a long time will need specialist cleaning but you should only consider this if you have learnt the correct techniques (see the links on my Roman page).
Never use abrasives or chemicals and NEVER polish coins. Polishing is the worst thing you can do. It makes them look superficially nice but you'll often find that you've just completely destroyed any value they may of had.
Collectors always shy away from polished coins, preferring coins that are in good original condition. Especially with regard to silver coins, I have found collectors usually prefer the dark, sometimes black tarnish to the bright 'Look at me I've been cleaned beautifully' coin.
It's all about our concept of how 'age' should look. A coin that is 200 years old and in good condition should still look 200 years old. If it doesn't look new, It'll never look new whatever you do to it. That is its secret and also a good degree of its charm.
Has this been useful? If so, please put a comment in the Forum