Bellatrix is right. Sulfur is key. I haven't tried the egg method yet, but Liver of Sulfur is what jewelers use to patina metal. Smells just like eggs! It gives the metal a soft blue/gray/black color. I always let my metal go completely black and then buff off the areas where I don't want the patina. I use 0000 gauge steel wool in a circular motion. It will leave a very light scratch on the surface, but that can be buffed out further with polishing compounds.

You can purchase Liver of Sulfur from any jewelry supply store, like Fire Mountain Gems. If you use it be sure to use it in a well-ventilated area.

Many of the methods that someone else mentioned are probably industrial. I wouldn't use any sort of acids because, not only are they dangerous, but they will actually eat away at the surface of the metal.

Didn't someone also recommend letting your cat pee on it?

I make jewelry and use the boiled egg in the bag to oxidize Sterling Silver it works well