The best way to maintain the aluminum hardware on your boat is to wash it regularly with soap, and coat it with wax at least twice a year. If you don't do this, oxidation will sooner or later dull and discolor the surface of the aluminum.
While it's easy enough to revive neglected stainless steel, brass, or chrome with a vigorous application of polish, aluminum is more difficult to restore. You can polish it, and there are polish formulations available just for aluminum, but you should use these products with caution. Most aluminum marine hardware is anodized, and polishing can remove this surface coating, leaving the aluminum unprotected. (Read the fine print: most polishes say "Do not use on anodized aluminum.") Instead of polish, try scrubbing the aluminum with fine bronze wool and a powdered cleanser (Ajax, Bar Keeper's Friend). Then give the clean surface a heavy coat of wax. This may not restore the aluminum to like-new luster, but it will retard future oxidation.