Brass and copper are soft materials that need to be carefully cleaned. Usually all that's required is washing with warm water and mild detergent and drying with a soft, clean cloth. However, when brass and copper become tarnished, some creative methods need to be used that will remove the tarnish without damaging your brass or copper item.
Use a chemical brass cleaner like English Custom Metal Polish and Restorer or a commercial copper cleaner like Copper Glo or Wright's Copper Cream. Make sure it's not an abrasive cleaner--you don't want to scratch the brass or copper.
Make a paste by mixing 1 cup of white vinegar with 3 tbsp. of flour and 3 tbsp. of salt. Put the paste on the copper or brass, rub it in and let it dry. Rinse it off with cold water and polish it with a soft, dry cloth. If you're out of vinegar, you can use lemon juice.
Soak brass and copper pieces in 2 cups of milk mixed with 2 cups of warm water. The lactic acid in the milk will remove the tarnish.
Mix 1 cup of lemon juice, 3 tbsp. of cream of tartar, and 5 drops of orange oil to make a paste. Put latex or plastic gloves on and rub the paste into the brass or copper. Let it dry. Rinse the surface with warm water and dry it with a soft cloth.
Soak the brass or copper in Coca-Cola for two days to fade and loosen the tarnish. Use a soft toothbrush and Brasso Cleaner to remove the tarnish.
Rub olive oil into the brass or copper in a circular motion with a soft cloth after cleaning to keep tarnish away.
Spray a coat of clear lacquer like Krylon onto the brass or copper to protect it after tarnish is removed. Wait 5 minutes and add another light coat.