Aside from a well-controlled environment, nothing contributes more to good tuning stability than frequent tunings. The whole idea behind tuning a piano two or more times a year is to catch it before it wanders too far off pitch. This way the tuner spends less time making large adjustments in pitch and more time tweaking and fine-tuning the instrument. Smaller adjustments in string tensions means better tuning stability. The piano stays in tune better between visits from your tuner.
It's difficult to explain but a piano just sings better when tuned to proper A440 pitch. The scale design of a piano was engineered to perform under specific tensions. If your piano's pitch is inconsistent and/or is sitting somewhere around a half tone flat it sounds dull and lifeless. If your piano is in good structural condition always be sure it is being tuned to A440.
More frequent visits from the piano tuner means he can keep a better eye on little issues that might come up before they become bigger ones. For instance, action regulation changes somewhat when humidity levels change. Eventually, action parts and pivot points can loosen over time and if not addressed will greatly accelerate wear on action components. JD Piano Service takes care of many of these adjustments free of charge during the tuning visit for customers on a regular service schedule. Little issues are easy to fix, bigger issues can become costly and most can be avoided all together with routine, scheduled maintenance.
Pianos that have been left untuned for several years tend to drop in pitch somewhere around a half tone flat. When raising the pitch, particularly on older pianos it is not uncommon to break strings. Strings can be replaced but it can get costly in a hurry and any new string installed will continue to stretch and be very unstable for quite sometime. Just like tuning a guitar, it takes several tunings to stabilize a newly installed piano string. Problem is, most of us can tune a guitar to some degree ourselves. Pianos on the other hand need a tuner and it's not convenient or cost-effective to call the tuner back each time your new strings wander out of tune. Again, more frequent tuning means smaller adjustments in string tension. Old, brittle strings are far less likely to break if they don't have to be stretched too much to bring back in tune.