To keep the value of your collection safe, here are four standards you should follow:
- Keep them cool and dry. Sharp changes in temperature and moisture—as are natural in most attics and basements—promote tarnish and spots that devalue your coins.
- Use original holders. All modern proof sets and commemoratives should be bought and sold in their original cases and capsules.
- Save documentation. A Certificate of Authenticity and an information card that come with United States Mint proof, uncirculated, and commemorative sets. If these are missing, your coins will be harder to sell.
- Keep them safe. There’s no safety like a safe-deposit box at a bank. If you keep your collection at home, check that your home insurance covers it for the full replacement cost.
- Flips have two pockets—one for a coin, the other for a label. Flips made of Mylar® and the like are good but can be a little stiff and brittle. When inserting or removing a coin, “bow” it by pressing opposite sides inward to avoid scratching the coin.
- Soft plastic holders aren’t always better. You definitely want to avoid those that contain polyvinyl chloride (PVC), a chemical that softens plastics. PVC can eventually coat a valuable coin with a sticky green slime, which eats into the coin’s surface and ruins it.
- 2″x2″ cardboard holders.
- Plastic tubes (ideal for rolls of coins).
- Hard plastic holders (preferable for more valuable coins) are available for individual coins and small sets.
- Polyethylene sleeves are good for short-term storage.
- Paper envelopes are okay for circulating coins, if the envelopes are made for holding coins. Other envelopes could contain chemicals that cause your coins to change color over time.
- Foldout albums can protect coins from wear and handling, but not from air-borne chemicals that cause toning. These albums are not a good choice for long-term storage of higher-grade coins.
- Slabs (sonically sealed hard plastic holders for individual coins) offer good protection, but slabbing is an expensive process, so it’s best to use it for more valuable coins.
Source: The above information was copied from the United States Mint website.