Being a bit of a car guy, I usually have on hand a small bottle of Griot's Fine Hand Polish, email ordered from Griot's Garage at:http://www.griotsgarage.com/
This is a much finer cut and much less abrasive than regular automotive rubbing compound, and is designed for removing fine swirl marks, etc., from automotive paint. I find that it works quite well on my accordions, applied with a nice, soft old bit of terry towel. I assume that other manufactureres, such as Mother's, Meguiar's, etc., will have similar products, but I would always test a drop on a hidden area before diving in.
As for the final shine-up, I use Dr. Duck's Ax Wax, from House of Musical Traditions:http://www.hmtrad.com/catalog/winds/sbx/drduck.html
This combination keeps even my black accordion looking quite nice!