If its brushed aluminum, use mothers mag/aluminum polish. Put a little on a terry cloth and go back and forth with the brushed grain. When the polish turns grey, wipe it off with a paper towel. Before I ride, I usually whip the bike/exhaust down with some glass cleaner and a paper towel so the heat doesn't melt anything to it. If it smooth aluminum then don't use mothers mag/aluminum on it, it will scratch it, use something like mothers chrome polish or something of the like. Just keep moving in the same direction so nothing grinds wrong and scuffs it up.