Don't ever clean copper using chemicals. On the other hand, you should keep your copper free from harmful contaminants. To do this, you should get them out about every 6 months, and carefully brush them off with a soft camel's hair brush. Silver Silver coins oxidize with fair rapidity. Particularly in the presence of sulfur.
Sulfur is present in some quantity in almost all environments.
The most common source of sulfur is paper products. Do not store your coins with any type of paper that you don't know is sulfur free. Most 2x2s and paper envelopes that you can find at a coin shop will be sulfur free, but it still doesn't hurt to ask. Envelopes from other sources are highly suspect. Very dark coins are generally considered to be undesirable.