I am a fan of the Blue Magic metal polish cream. I use this to keep my exhaust shiny on my BMW.

For all metal surfaces
Non-abrasive
Great on brass, copper, sterling silver, aluminum, stainless steel or gold
Removes tarnish and oxidation
Can be used with buffers and polishers
Provides lasting protective coating
http://bluemagicusa.com/index.php/blue_magic/products/210/blue_magic_metal_polish_cream_jar/