Copper in its pure state will change colour over time, becoming generally darker.
A quick rub with a soft cloth should buff it back up, or if you prefer, a bath in some diluted lemon juice will bring back its original shine.
Remember to only leave in the lemon juice for a minute no more, and dry thoroughly.
For delicate pieces, a gentle brush with some dish soap followed by a thorough rinse and dry will also work.
When storing your piece, an airtight bag will help slow down the darkening process. But for most people it is your own particular preference; some believe that copper is a living metal that develops its own colour to suit the wearer.