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A lint-free cloth that's slightly damp (tears of envious friends work well).
 
Windex Wipes

wipes.jpg
 
the apple store.

lol no really, if you bring it in for repair they usually clean it, and mine was in for some reason every 9 months.
 
3 parts 99.98% Isopropyl alcohol. 1 part water, and a very fine fiber cloth. Kleenex works for the cloth. Most terry-cloth towels aren't suitable for aluminum if you wish to keep that original glow it had when new.
 
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