Real brass (as opposed to brass plated) fittings and ornaments are usually coated with a clear lacquer after manufacturing since brass darkens and corrodes rather readily. To check for the lacquer coating simply wash the fitting with warm, soapy water and dry with a soft cloth. If lacquered, it will be shiny and your problem is solved.
If, on the other hand, the lacquer has worn through and the fitting is darkened considerably, try enough lemon juice to make a paste out of some baking soda. Simply apply the paste, rub with a soft cloth and then rinse with warm water. Friends say they get good results from half a cup of vinegar mixed with a palm full of salt, but I've not tried that... Best of luck!