you just go with finer and finer water paper, until you get to the 1600 grit. at that stage it starts looking like a mirror finish.
then all you need is some metal polish, and you can rub it down, or you can get a polishing wheel attachment for your drill, and some of those dry polishing compounds. dont push too hard though, i bent the shaft of my drill by polishing my tappet covers and various other bits!
