If you have sterling silver that is starting to oxidize, either baking soda or toothpaste on an old toothbrush works really well to remove the darkness.
If I have a chain, I will pour baking soda in my hand, drip a little water to make a paste then run the chain through my hand several times.
To help keep sterling from oxidizing, keep in a plastic bag with a dessicant, such as the little packages that come in shoes. This will help keep moisture away from the jewelry. Just keep it away from little ones! When my son was about two, I caught him about to eat one! At least I hoped he was about to eat one! One call to the poison control center later, I threw every one I had away!