The best way to keep your piano healthy is to tune it at least once per year. With the passage of seasons, temperatures fluctuate which causes wood, iron, and steel to expand and contract at different rates. These subtle changes can loosen piano strings over time and lead to half a semitone drop (or more) in tuning. Professional and serious musicians often tune their instruments more frequently to stay in tune with their fellow musicians and to train their own ears to hear pitch correctly. If this is you, consider tuning your piano once per season, especially before group sessions and/or public performances.