On cast aluminum (which is more difficult to get to a high luster than billet) I always did 600 grit wet, then immediately switch to #00, then either #000 or #0000 (finest) steel wool.
From there you only need some good polish, I swear by Mother's. They have a Mag and Aluminum polish, which works wonders on plastic car headlights, and even lexan helmet visors. For the ultimate gleam you'll want their Billet polish, which is a finer blend.

That reminds me is this is what I need to do to my 4runner head lights. The first time I saw this my friend did it to his mustang and it made them look new and clear again.