For short term and long term stability
The wood in pianos, located in the keys, action parts, bridges, soundboard, and pinblock, contains about 3-4% moisture. One of the main factors in de-tuning is humidity. Attention should always be paid to the air vents in the room to reduce the amount of direct air flowing onto your piano. Always avoid direct sunlight. Even indirect sunlight in a room for an hour will alter the humidity. Open doors and windows will introduce humidity shifts that can alter a tuning. A hygrometer is highly recommended to keep a record of the humidity near the piano. If more stabilization is desired, a dehumidifier and/or humidifier for the room is recommended. For the most thorough humidity control, a system called Dampp-Chaser can be installed in your piano. Ask your technician about the pros and cons for each option.