The secret to keeping your aluminum looking new is never to let it start looking old. This is easy to do by following the three basic rules of aluminum maintenance: "Keep the salt off, keep the salt off and keep the salt off." These rules apply equally to any anodized aluminum, including your windshield, salon windows, rails, bridge ladders, etc. Clean the metal every time you use your boat. Do this not by standing in the cockpit playing a hose stream over the tower, but by washing the aluminum with soap and water, then rinsing it thoroughly. A foam mitt makes the job go faster.
If you have an open frame hardtop or half tower, don't forget to wash the carlines - the aluminum beams - supporting it. Outrigger poles need the same care; clean the salt off the butt of the pole (the part that sits in the base) and out of the base itself, too.
Every month or two or three, depending on how you use your boat, after cleaning the metal, protect it with a high-quality paste wax or, much easier, a wipe-on product like Woody Wax (www.woody-wax.com) or Rupp Marine's Aluma Guard (www.ruppmarine.com) Wear a cotton glove, dump some in your palm and rub it onto the metal. There is no easy way to bring the anodized finish back if you let it go bad. Restoring the the brilliance of the finish will take many more hours than maintaining it.
But if you've neglected your tower or hardtop and corrosion has started already, don't despair. The damage is likely to be limited to the anodized coating; the underlying aluminum should be O.K. Aluminum workboats or hard-core bluewater sailing yachts are often left unpainted; oxidation creates a hard, dull grey coating that protects the underlying metal better than paint would. But if you don't want your Tower to look like that, you can do a pretty good job of restoring the finish of a "greyed out" tower if it was properly anodized originally. Start with strips of fine sandpaper, no coarser than 600, using them "shoe shine" fashion around the pipes, then work on up to 1200 paper, then a strip of cloth, with rubbing compound, a final polishing with something like Flitz, and then a good coat of Woody's wax applied frequently.