Bronze, copper, brass...very difficult to do anything with. Depends if it is ancient and uncleaned or fairly recent (1800 +). Generally water, acetone, perhaps some soap, but make sure to THOROUGHLY rinse it off.
Are we talking about gunk or staining? For gunk, start with distilled/purified water soaks. Work up to acetone soaks. For staining, there really isn't much you can do. VerdiCare will help stabilize the surface of the coin to prevent further damage.
It would help to have a picture of what you are working on. I have seen some amazing results cleaning copper and bronze coins, but it's really only very specific problems that can be helped.
