How silver tarnishes
Tarnish, which appears as black or brown streaks and patches on silver ware, is silver sulfide, a compound formed when silver interacts with sulfur in the air. Sulfur is a byproduct of burning fossil fuels and was a constant concern of homemakers when houses were heated by coal furnaces, and silver items not used on a frequent basis were polished at least once a month to keep their shine.
Commercial ways to prevent tarnish
Commercial silver polishes employ fine-grit abrasives, ammonia and other ingredients to dissolve tarnish. Some contain surfacents that retard future tarnishing. Other metal polishes and toothpaste (a popular emergency substitute) contain abrasives too coarse to be used on silver, leaving scratches that cannot be removed and permanently dulling the surface.
"Silver-cloth" contains tarnish-retardants. If you have favorite but seldom-used pieces of silver, wrapping them in silver-cloth will lessen the tarnish they accumulate.
Home remedies for tarnish
The best way to prevent tarnish is to use your silver flatware and serving pieces often. Regular washing and drying come with regular use, keeping silver shining. Acids, like canned tomatoes, and sulfur-containing foods, like egg yolk, do not have the opportunity to accumulate and discolor your silver. Silver-plated flatware and serving pieces are in danger of being polished enough to remove some of the silver-plating. An easy alternative is to fill a large saucepan with water; and place a sheet of aluminum foil in the bottom of the pot unless the pot is already aluminum. Bring water to a boil, turn off the heat, and add 1/4 cup baking soda. The resulting froth will remove tarnish without rubbing. Reheat water and/or use more baking soda as needed. Rinse, dry and enjoy the shine.