Silver polish and a lot of rubbing.
Sometimes you can put a little TSP in a glass or plastic bowl (not metal), add a sheet of aluminum foil then put a couple pieces of silver on top, in direct...
Get some Brasso and start polishing. NeverDull (a wadding polish) does a fairly decent job, too. After you finish, wipe all of the residue off with paint thinner and put a clear polyurethane spray...