This is what I do to maintain my accessories and they really last :
Oxidation causes accessories to tarnish. Never hang your accessories up on accessories stands though they may look pretty. This cause them to tarnish easily as they are in constant contact with oxygen/moisture. A good and cheap tip (mentioned by Crystaldollies) is to keep them in ziploc bags. I always buy mine from Daiso, they come in varying sizes to suit different needs - e.g. small ziplocs for earrings, bigger ones for chunky necklaces. I keep one piece of accessory to 1 ziploc to avoid scratches etc. This also makes it easy for me to look for a specific piece s ziplocs are transparent.
Avoid contact with oils, salts or chemicals. Do not handle your accessories when you have just applied hand cream as it contains chemicals - another one we all know is perfume. Wait for awhile after you've put on hand cream before you put on your rings and remove your rings before washing hands with soap. Wipe your accessories with a soft cloth after wearing to remove the oils and dirt.