I spend a lot of time checking all my pieces for any signs of tarnish. Jewelry that I may have for a few months before they sell may have some tarnish on them from exposure to the environment. Tarnish is actually is oxidation due to exposure to the air. Many of my pieces are also exposed to a lot of handling by interested customers. Fingerprints seem to speed up the process.
Silver pieces will definitely tarnish over time but did you know that plated pieces can also tarnish? You could use a silver polishing cloth on your jewelry, but this takes quite some time and elbow grease. I've spent some time researching this issue looking for a procedure that would use everyday household products. This is what most people recommend:
1 tsp baking soda
1 tsp salt
1 cup water, hot not boiling
1 sheet of aluminum foil
Cover the plate with the foil. Place salt and baking soda on the plate. Pour the water over the powder and salt and carefully mix them together.
Place the jewelry in the watery mixture and wait a few minutes. The foil pulls the tarnish from the jewelry onto itself.
Rinse, towel dry and, using a soft cloth polish the remaining tarnish from the jewelry.
Let me know how it goes!