Copper tarnishes over time, especially when not being worn. To prevent premature tarnishing, keep in a plastic, away from air and moisture.
TO CLEAN and bring back shine:
You may use vinegar/water/salt OR lemon juice/water/salt - swish jewelry around in solution for 1-2 minutes, rinse with water . It's safe for stones too.
Sometimes you may not be able to submerge the jewelry in the solution. In this case, you may apply ketchup to the copper to clean. Let it sit fir a minute or 2, then rinse and dry thoroughly.