A mini angle grinder with sanding discs or similar to start. If you've used a grinding disc then you will prob have to start with around 40 or 60 grit, a flap disc would also work. Then go up through the stages, 120,240 etc to about 300-400 grit. Then you need a coarse polishing soap like Supercut 40 on a sponge or similar compounding mop. Then you can use something like autosol just to bring up the shine and finish it off.
It needs to be spot on mirror finish if you don't want it rusting.
At work we get all our polishing supplies from here.
Their site is worth a good look, they have some handy guides etc to help you with polishing metals.
Edit - website is having an upgrade, polishing guides not there anymore
METHOD No. 2 - pay someone else!