I use a six-hundred grit wet/dry sandpaper, followed by a 1200 grit paper (I work at a place that does a lot of metal work, so they have this stuff around), and then finish it off with Mothers Aluminum Polish.
What I said above is all well and good, but most important is elbow grease. Turn on the music, a DVD, or just get into the zone and keep polishing (and polishing (and polishing)). I've had to put an hour and a half into polishing a stem and another time, since it was an already polished frame that just had surface oxidation, only an hour for an entire frame.