You can avoid tarnish by wearing your jewellery often, as the oils on your skin will "clean" the silver and keep it looking shiny. Unfortunately, the metal will still become dull or darken with normal wear. This is because lotions, direct sunlight, soap, detergents, perfume, contact with household chemicals, salt water, chlorine in swimming pools and even perspiration all take their toll on fine jewellery and tend to accelerate tarnishing. Take off your jewellery before doing household chores or gardening to avoid knocking out stones, and it's a good idea to take off your jewellery before applying creams and lotions. Gemstone settings can get clogged up with cream easily.
To avoid tarnishing when you store your jewellery, make sure it is stored airtight, as humidity is an enemy to precious metals. Your bathroom is the most convenient yet the worst possible place for you to store your jewellery. You can use little ziplock bags. Do make sure you don't store multiple jewellery pieces in the same bag; the individual pieces can scratch each other. Keeping link or chain bracelets unclasped or unhooked will prevent scratching as well. If humidity is a problem, you can place a piece of chalk, a packet of activated charcoal, or a package of silica gel in your jewellery box.
Jewellery should be the last thing you put on before leaving the house and the first thing to take off when you get home!