As Ralph writes, cleanliness is key. I'll add that speed is important too -- clean the piece and, as quickly as possible, get an impermeable coating over it. I've been told that warm distilled water is the proper final rinse but can't verify that. Copper is notoriously reactive and is pretty indiscriminate about what it bonds with so you want to keep the rinse as pure and neutral as possible. Lacquer is the traditional shine-preserving coating and I'd think that most poly would be too thick to look right.