Have your piano tuned every year. Due to the tremendous amount of tension on the strings, pianos are constantly going flat. Playing the piano makes it go flat a little faster, especially in the middle section where you play most often. For this reason, heavily played pianos need tuning every six months.
There are some other occasions where a tuning may be necessary, for example when you move your piano. The simple act of lifting, setting down and rolling disrupts the careful placement of its 236 tuning pins, and humidity and temperature variations in its new environment will cause it to go out of tune.
In fact, just moving the piano to different areas within your home can cause it to slightly lose its tuning, especially if it is to or away from a window that gets direct sun, a sliding glass door to the outside, an air-conditioning vent or any other area that experiences hot and cold shifts throughout the day.
If you are a recording musician, it is absolutely essential to have the instrument tuned directly before a recording.