You can protect items from tarnishing by keeping them away from oxygen. You can do this by lightly coating the jewelry in wax but that solution is somewhat extreme.

I think the best way to maintain such items is to store them in a dry, tightly sealed environment. Even when sealed away, however, items will still tarnish over time. For extra protection you can place oxygen scavenging packets in the container with the item. If you live in a humid region you might want to also put some water vapor scavengers (silica packets) in there too.

http://www.drypak.com and similar companies carry the items you would need for this.

Of course, there are acid solutions available that gently remove tarnish should it occur.
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