Final brass polish
After you have the major tarnish and oxidation removed with Brasso or citric acid, try ordinary lipstick for a final polishing compound. Rub it on a brass part and polish with a soft cloth and elbow grease. It gives a fantastic polish that lasts for, oh a week or two, until the brass starts tarnishing again from natural oxidation.
Jewelers Polishing Rouge is made from the same compound as lipstick.