Copper, left to the elements, even indoors, will eventually tarnish. It will turn a bit black, perhaps streaked and then although it may take as long as 15 years, it eventually turns that beautiful green that we see on church roofs. If you are trying to keep a copper mailbox or decoration in good as new shiny condition, you will have to completely clean it with a copper cleaner then spray it with an acrylic or lacquer finish to keep the oxygen away from the copper. It really works, and saves regular jobs of elbow grease.
If you have copper that someone cleaned too much and you want to get back that antique look, then apply some muriatic acid or even straight lemon juice. The lemon juice works slower but a bit more evenly, to initiate that aging look overnight. In fact, anything that cleans copper, if left on too long will age it.