What will protect my copperware from tarnishing

How can I keep my freshly-polished copperware from tarnishing so soon?

if you are using nickel lined cookware and are planning to use it in the future for cooking, do nothing to treat the ware. I use a product that removes the oxidation almost immediately with a slight scrumming the a cheap nylon scouring pad, the cleaner is called Zap Restorer ⇦ on eBay or Amazon [affil link] and comes in a purple spray pump.
If the copper ware is purely decorative the you can clean it throughly, dry it with a soft cloth that will not scratch and apply a thin coating of clear polyurethane, that is easily removed with an acetate solution. The polyurethane can be diluted with a 1/3 mixture of mineral spirits ⇦ on eBay or Amazon [affil link] and the drying time is greatly reduced and goes on much smoother.