1. Simichrome polish. I learned about this German-made metal polish 40 years ago when I had a motorcycle with lots of aluminum parts that tended to get dull. I've never found anything better. Squeeze a tiny bit on a soft cloth, and rub the aluminum surface. It will remove oxidation and leave a soft lustre. If you keep rubbing, you can actually create a mirror finish! Great stuff, and it used to be reasonably priced, but I think a small tube now costs several dollars. I've seen it those swanky spots car accessory shops.
2. Nevr-Dull wadding polish (yes, that's the correct spelling). This product comes as a can filled with cotton wadding that's impregnated with a polishing compound. It's widely sold in hardware and housewares stores as a polish for brass fixtures, silverware, etc. You tear off a bit of the wadding and rub the metal until the oxidation or tarnish is removed, then you rub the piece with a soft cloth to buff it. You don't have to rub hard in either step.
I bought a can of Nevr-Dull ($5.99) last year to try on the tarnished, slightly rusty chrome plated frame of a 50-year old vibraphone. I couldn't believe how easily it removed the tarnish and even most of the rust. When I saw your question here, I ran down to the basement and tried Nevr-Dull on the oxidized 1982 SunTour Cyclone II derailleurs of my Trek restoration bike. Some of the surfaces originally had a dull finish, and now, they look smoother and the color is consistent. The parts that originally had a brighter finish polished up in seconds. They really glow! The chrome-plated derailleur hanger bolt glistens like new. I used just a pinch of the wadding, so I think it would be really economical to use.
I'm going to try this stuff on an inconspicuous place of a vintage Sugino GT crankset I bought for the Trek. I want a soft, patinata look, not overly bright. It may take some self-control to keep from going too far in polishing.
Neither of these products should be used on surfaces that have a lacquer coating or on painted areas. I guess that means you can't polish parts that have the maker's name stenciled or silk-screened onto them without damaging or removing the lettering. Otherwise, they really easy to use.
EB