Most tarnished brass items can be cleaned at home with very little effort. Commercial products are available at any grocery or hardware store for this purpose, or you can make your own cleaner with simple household ingredients. If the item is an antique or heirloom, a qualified brass restorer should perform the cleaning so that the value of the piece is not affected by the cleaning.
Using a Commercial Cleaner
Wash the piece in hot, soapy water to remove dirt and grime. Dry thoroughly.
Dip a cloth in Brasso or other commercial brass cleaning product. Rub the tarnished area with the cloth until all tarnish is removed.
Rinse under clear, warm water to remove any cleaning residue. Dry completely with a soft cloth.
Using a Homemade Cleaner
Fill a sink with hot, soapy water. Add 1/4 cup of ammonia. Dip the piece in the water and rub gently with a soft rag to remove dirt and grime.
Dip a soft cloth in regular white vinegar. Dip the wet cloth into a bowlful of regular table salt. Gently rub the vinegar/salt solution over the tarnished area. Re-dip the cloth as often as necessary until all tarnish is removed.
Rinse the tarnish free piece under fresh, hot water and dry completely.