When that happens, a quick buff with some soft cotton cloth will work wonders. Or, if you don't have the patience or the elbow grease, use a polishing cloth.
Don't be tempted to use a dip without reading the fine print. For the most part they will harm whatever else is on your jewellery (pearls, semi-precious stones), and if you don't rinse the piece properly residue will remain. And it's hard to get it off. And whatever you do .. don't use a toothbrush!
Things that will help prevent tarnishing:
* Don't wear your jewellery when doing housework - or to the spa.
* Store your jewellery with small anti-tarnish strips in small bags (or a box)