Someone mentioned vinegar, so I'll chime in. I've had good luck soaking assorted metals in vinegar to remove oxidation. I believe the free hydrogen ion in the acid bonds to the oxygen atom of the oxide and strips it off the metal. (I'm working with some pretty "rusty" chemistry knowledge here.)
Anyway, I just cleaned a set of brass window locks by soaking them in warm vinegar for a couple of hours and then giving them a good brushing with soapy water. They came out a bit "coppery", but OK.
Years ago, I use to soak brazed welding wire sculptures to remove oxides formed during the brazing. The procedure was to soak overnight at room temperature and brush as above. They came out nice and shiny.
The only thing I would caution is that I believe the item should be fully immersed in the vinegar. I'm not sure what would happen at the vinegar / air interface if the part wasn't fully immersed, but something tells me it would be unacceptable.