Basic Piano Maintenance
Two of the most important things that you can do for your piano are to keep it in tune and maintain a constant ambient humidity of about 42% (which will maintain the wood of the piano at 8% moisture content).
If the piano is tuned regularly (and the room humidity is controlled), the instrument will eventually settle into an equilibrium state where only minor adjustments are needed at each tuning. New pianos typically require about 4 tunings in the first year, and then at least 2 per year thereafter. I find that even after the first year it is best to touch up the tuning after every season change, i.e. every 3-4 months. Pianos that have not been tuned for a long time will be so far out of tune that they will most likely require a “pitch adjust”. This is an iterative process where tuning is repeated until a stable tuning is obtained. More information on the procedure can be found here
The primary reason that pianos go out of tune is a change in humidity. The wood of the piano absorbs moisture from the atmosphere. Changes in the water content of the wood lead to expansion or contraction of the wood and dramatic changes in the tension on the strings, thus a change in tuning. The total tension on the strings of a piano is about 18 tons, or about 150 lbs on each string. The slightest change in tension on a string can cause a pronounced change in tuning. Maintaining a constant humidity in the piano room is the easiest way to maintain a good tune. Opening the windows and doors when spring arrives can be very attractive, but the tuning of your piano will suffer.
In some situations regulating the humidity of the room or space where the piano resides can be impractical on a continuous basis, e.g. in a church sanctuary. In those situations a Dampp-Chaser Piano Life Saver System is an option. An example of the installation and performance of such a system in a Steinway D is given here
Further information on piano maintenance can be found at the Piano Technicians Guild website.