Tarnish is a natural and inevitable process which occurs to sterling silver, Old Sheffield Plate and electroplate (silver plate). The time silver takes to tarnish depends entirely on the amount of humidity and pollutants in the atmosphere.
Regular cleaning and polishing will restore the shine to silver and will help prevent tarnishing. Similarly, measures can be taken to preserve the lustre of objects not on permanent display, or regular use. These can be kept in tarnish proof bags available widely. Another method is to wrap articles in acid-free tissue paper and place them in plastic bags, making them as airtight as possible before storage.
Do not be tempted to wrap silver in cling film (plastic wrap); certain brands cause a chemical reaction which can permanently discolour the silver.