If they really are plain brass then they can be polished up to a high shine either in situ or best, removed - taps are relatively easy in comparison to say, a wire basket.
If they are just yellow metal with a thin, plated coating of yellow alloy covered in hard laquer - once the top coating is damaged you are best off replacing if "look" is important to you as you will never restore them without an expensive industrial process costing more than new taps.
In situ polishing of brass requires a lot of effort. Metal polish, old toothbrush, very fine wire wool and polishing rags. A process that will need to be repeated quite regularly by your domestic staff to maintain the shine.
Metal polishing on a buffing wheel can produce excellent results but you have to remove the item to be polished to do it properly, see...
http://www.youtube.com/watch?v=lIsc1nDzLak
Chrome plated bathroom fittings are less demanding. Never tarnish, rock hard coating, high gloss cleaning with minimum effort.
Peccavi, December 2008
