Sterling Silver  becomes tarnished when exposed to humidity. When not wearing jewelry store in a moisture absorbing cloth. I have found cotton to be the best. I store my collection in cotton bags in a box with moisture absorbing crystals of silica jell. You will find packets of this stuff in some items that are shipped.

The quickest safest way to clean gold or silver jewelry is to use a fluoride toothpaste and an old toothbrush. Squeeze a little toothpaste on the jewelry, wet the toothbrush and scrub the jewelry. Scrub with the toothbrush all over the jewelry for about a minute. Then rinse jewelry under running water with the toothbrush until the jewelry dose not foam anymore. Dry the jewelry with a soft cotton rag. I clean my collection all the time with this method as well as pieces I'm delivering to the new owner.

For jewelry that is really tarnished (a slight brown to black color) I apply the toothpaste and let the jewelry sit for five minutes before I scrub. If there is still tarnish I repeat the process until it is shinny again.