The oft-repeated advice is to never clean coins and that is absolutely true if you don't know what you're doing - you can irreparably damage a coin by trying to clean it. If you think a coin needs cleaning, get some advice before doing anything. Always try on a low-value coin first just to make sure.
Avoid chemical cleaning of copper/bronze coins at all costs. Stick with a tooth-pick or something else that won't scratch the coin's surface.
A small amount of white vinegar applied to any difficult to remove dirt or verdigris should loosen it up or remove it.