Have your piano tuned regularly. Twice a year minimum.
Ask your technician to do a minor "touch-up" regulation at each tuning. This will prevent most instances of unnecessary wear and breakage.
Have a full regulation done every 2 to 5 years. You'd be surprised at how your piano should have sounded and responded to you playing all these years.
Don't attempt any "home repairs" on your piano. Although it may appear easy to fix yourself, an innocent mistake can be costly. Let a technician do it - they have the right tools, replacement parts and expertise to do the job right the first time.
Keep your piano away from heating registers, radiators, fireplaces and air conditioning vents. Subjecting it to extreme fluctuations of temperature and humidity levels can do major damage. Keeping a piano away from an outside wall was probably necessary in poorly insulated older Victorian homes, but should not be a problem these days. Avoid direct sunlight on your piano - it can damage and fade the finish, or even create horrible tuning problems. Use curtains or blinds.
Try and keep the temperature and humidity levels as consistent as possible in the room where you have your piano. Using a temperature/humidity gauge (hygrometer) can keep this in check. Seasonal swings in relative humidity are the piano's greatest enemy. Swelling and shrinking of wooden parts affect tuning and keyboard touch, while extreme swings can eventually cause wood to crack and glue joints to fail. Pianos are happiest in a relative humidity level of 40 to 45 percent. If you don't have a central humidifier/dehumidifier, consider getting a room humidifier for the winter, and a room dehumidifier or air conditioner during the summer months. There's even such thing as a special piano humidifier/dehumidifier that can be installed in your piano. Ask your piano technician.
Keep plants, vases, drinks, or anything to do with liquid off the piano. Condensation can ruin the finish, and spillage of liquids into the inner mechanism can result in irreversible damage.
To prevent scratches on the finish, never place objects on your piano without a soft cloth or felt.
Don't use furniture polish to clean your piano. It can soften the finish if overused, and the silicone & oils present in many household brands can even contaminate the wood, despite what the labels may say.
Just feather-dust the piano first (dust is abrasive, so wiping it first can cause scratches), then wipe with a soft, damp cotton cloth, wiping in the direction of the grain. Then, wipe up any excess moisture with a similar dry cloth.
Do the same thing as above to clean your keys, or just a small amount of denatured alcohol, but use separate cloths for the blacks and the whites. Avoid any liquid running down the sides of the keys. This is fine for ivory or plastic key tops. Don't use cleaning agents!