You'll need to first start off with sandpaper to work out all of the deep scratches in metal. I usually start with 220 unless there's some serious scratches. Once all of the scratches are gone, switch to progressively finer sandpaper (320, 400, 600, 800....) By the way, I usually wet-sand, and use a little bit of dish detergent in the water to keep the sandpaper from clogging.
You'll find that as you get to finer and finer grades of sandpaper, you'll be approaching a mirror finish. However, to get the most brilliant shine, use a buffing wheel or a vibratory tumbler with corncob media. Either way you can get a mirror finish on aluminum, which will look like chrome.
Check out www.caswellplating.com They've got a good HOWTO on buffing aluminum, and they also sell buffing supplies.
Here's a pic of a piece of polished aluminum. You can get a much flatter surface than this (notice the machining swirls), which will yield a better reflection.