The enemies are humidity, air and handling
Most damage to coins comes from moisture, from chemicals in the air and holders, and from careless handling. Humidity speeds up corrosion and toning, so a cool, dry room with a steady temperature is better than any single product.
Avoid basements, attics, kitchens and bathrooms, where humidity and temperature swing the most.
Choose inert holders
Use holders made from inert materials such as Mylar flips, acid-free cardboard, or hard plastic capsules. Avoid soft vinyl flips containing PVC, which can leave a green sticky residue on coins over time.
Third-party grading slabs are a good option for higher-value coins, since the holder protects the coin and records the grade.
Handle by the edges
Hold a coin by its rim between thumb and forefinger over a soft cloth, so a slip does not become a drop. Never touch the faces, because skin oils leave fingerprints that can etch the surface over time.
Do not talk or breathe over uncovered coins. Cotton or nitrile gloves are fine; just be careful, since gloves can reduce your grip.
Cabinets, boxes and safes
A fire-rated safe or a bank deposit box protects against theft and fire, but sealed spaces can trap moisture. Add silica gel packets and replace or recharge them regularly.
Keep coins in an organised order: by country, date or set. Label every holder in pencil on the card or on an outer slip, rather than writing on the holder.
Insurance and records
Photograph each coin clearly on both sides, and keep the photographs separately from the coins. Record cost, date, grade and certification number. Many home policies limit cover for collectibles, so ask your insurer whether a separate schedule is required.
Your holdings on Kollektra can store the details that insurers and executors often ask for.
Travel and display
If you carry coins to a show or a dealer, keep them in your hand luggage, in a padded case, and keep the case out of sight. For display, use closed cases away from direct sunlight and strong lighting, which can fade toning and warm the metal.
Rotate displayed coins and check them regularly, so a small problem is caught early.