Treat your binoculars carefully and they will allow you to scope out distant ice lines, fearsome rapids, and elusive wildlife or for years to come.
Non-waterproof models should not be exposed to excessive moisture. On wet or rainy days, carry them in their case outside of your clothing. In very wet weather, keep them in a plastic bag inside their case when you're not using them, or leave them in your pack during rain showers. While it is tempting to carry them tucked in your clothing, this is a warm, humid place, and when you pull them out for viewing, the sudden temperature change can cause internal fogging.
The most common cause of reduced performance over time is loss of alignment due to shocks or vibration. Try to avoid knocking or dropping your binoculars. Realigning/collimating a damaged instrument is a costly proposition.
Do not submerge waterproof binoculars at depth or use them under water. The increased pressure can cause them to leak.
If they have flexible, roll-down eyecups, store the binoculars with the eyecups flipped up. The eyecups will suffer excessive stress and wear if left in the down position. Twist-up or slide-out eyecup models may be stored in any position.
Store them in a cool, dry place. In warm, extremely humid environments, store binoculars (even waterproof ones) in a sealed plastic bag with a small packet of silica gel (available from most camera shops). This will prevent the growth of mildew on optical surfaces.
Marine use and salt spray can be especially damaging to binoculars. When salt water evaporates, abrasive and corrosive salt is left behind and can penetrate unsealed components. You can immerse waterproof binoculars in fresh water to rinse off salt. Do not hold them under a tap, as the increased water pressure may force water past the seals. Gently wipe salt residue from non-waterproof models with a damp cloth and let them dry thoroughly.
To clean the lenses, use a cleaning solution and a lens tissue or a soft lint-free cloth. Sunscreen and oils can be removed with cleaning solution (or dish detergent and water) and a bit of polishing.