If you are doing it by hand, soaking it in ketchup is supposed to work but there are proper polishes meant for brass which you should be able to find at your local hardware store. I tried using Twinkle once meant for copper pots and pans but it was abrasive and scratched the metal. It sure took the tarnish of though!
If you are using a buffer, a rouge meant for brass will give a mirror finish to the metal. It may take two different steps using different color rouges. You just have to remember that the red jewelers rouge will color the metal a bit, making it a bit darker and that’s why I don’t like using it on aluminum. White rouge gives the best finish to aluminum.