This was one of my favorite threads on the old numismatic forum, so I thought I'd reproduce it here. I have more than one method of toning a coin, so I'll post separate methods along with results.
Take some liver of sulphur (you can get it from a jewelery shop or online), approximately one teaspoon or so. Add one part rubbing alcohol or some other alcohol that will burn when lit to every two parts liver of sulphur. Use a q-tip to apply a generous amount to the surface of a silver coin, let it sit for 10 seconds or so, and light it on fire to let it burn out. You get a beautiful blue tone out of this.