Trial and error with various storage mediums have demonstrated that coins can be easily damaged by chemicals in many papers and plastics. Acid free paper envelopes work well. Harder plastics seem to be much better than softer ones.
PVC plastic causes problems for many coins. Many metals turn green if stored in PVC, though it may not be the PVC itself as it is the other chemicals it is manufactured with.
Different metals react when in contact, this includes different types of coins. Even identical coins will scratch each other if loose.
Coin envelopes are good for long term storage; designed to be acid free and inert the coin survives well if the humidity is low. Silica gel helps. Unfortunately you have to remove the coin if you wish to examine it.
Vinyl pocket are good. Multi-pocket vinyl is especially useful as they allow several coins to be displayed together. A good idea for sets of coins.
Coin flips are a cardboard square with a clear, round window on each side for displaying the coin. They offer reasonable protection and allow the coin to be examined without removal from the packaging.
Coin snaps are hard rectangular plastic cases that preserve coins well from the elements. The coin is well protected and can be easily examined.
For all coins; – Adding silica gel packets to the storage medium (the type of thing that comes with vitamin tables) get rid of virtually all moisture. – Moderate temperature is always preferable. – Avoid exposing the coin to air, especially if they are mint condition. Coins tarnish easily in air. – Sunlight will damage coins quite quickly. – Avoid any situation where coins can rub together and get scratched.