Talk to an appraiser before you clean any coin. You may accidentally lose details that can determine the value of the coin. Sometimes it's the patina of the metal that helps determine the value, so don't clean it off. Never use Coca-Cola, it's an acid and can eat the metal and the patina or tarnish. Some people recommend using a weak solution of bleach or ammonia but I don't know how effective that would be. Again ask before you do anything to the coin, my sister thought she had a funny colored penny, turned out she had a steel penny from WWII which was fairly valuable, if she had cleaned it, or done anythhing to it at all it would have really messed up it's value.
What kind of coins? If they are very valuable, it is best left to the experts to do... if you just want to clean them for your own use, ketchup or Worcestershire sauce clean copper great, and a cut potato dipped in baking soda will clean silver well.