Rub steel wool carefully over the scratch. Go with the grain, and brush lightly so you don't damage the finish.
Wipe off the surface of the aluminum with a clean rag, and examine the scratch. If the scratch is still there, rub the steel wool over it again, applying slightly more pressure than before. Wipe down the surface again.
Buff the surface of the aluminum by spraying window cleaner onto a dry rag and wiping the surface where the scratch was.
Protect the aluminum surface by covering it with a protective film or case designed to fit over it.