How to clean brass
There are plenty of polishes on the market designed to clean brass.
You could try Brasso Metal Polish Liquid, Bar Keepers Friend Cleanser, Blue Magic Metal Polish Cream, or any other type of metal or brass cleaner.
To breathe new life back into the brass, you should use these cleaners regularly according to the instructions.
Never soak brass before polishing it, instead go in with a damp cloth and soapy water to get rid of the dirt and dust.