It is very easy to clean copper jewelry and keep it a beautiful copper color.  You don't have to have a store-bought cleaner, you can use things that can easily be found around the house in most any kitchen; vinegar, lemon juice, or ketchup!  (Click this link White Tiger Tracks Store to go to my  store where you can find great copper rings, necklaces and bracelets like my His & Hers Copper Baby Boomers Bracelets pictured below!)
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Vinegar

Place your copper jewelry in a glass, or plastic container, not metal.  I use a drinking glass.  Pour enough vinegar over it to completely cover it.  Let stand for 5-10 minutes.  Rinse.  Dry.  You're done!

Lemon Juice

Follow directions used above for vinegar.

Ketchup

This is my least favorite, as it is a little messy, but it will certainly do in a pinch!  Simply rub your piece of copper jewelry with ketchup on a cloth until it is shiney.  Rinse.  Dry.  You're done!

NOTE:  If you buy a piece of copper jewelry that has a protective lacquer on it, do not try to clean it by any of these methods.     Be especially careful if you have purchased a vintage piece as you could possibly decrease its value.  A lot of collectors sell pieces that have not been cleaned by them and leave the cleaning up to the buyer if they so desire.