The best kind of stool is a height adjustable stool, particularly if more than one person in the house plays.  Whatever kind of stool you have, sit at a height where your forearms and wrists are parallel with the floor, and level with your fingertips when your hands are in a playing position on the keyboard.

Position the stool far enough away from the piano so that your elbow are free, and slightly ahead of your body, but so that you can comfortably reach the keys with your hands.

Always keep fingernails short.  This means that you can bend your fingers, and support your fingers from your hands.  Find the ideal hand position for fluent and fast playing by putting your fingers on the keys flat, and drawing the tips of your fingers back so that your knuckles are raised.  The palm of your hands should make a cave-shape.

If you need to wear glasses or contact lenses to read music, make sure that you wear them!  Otherwise you may end up straining your neck forward (and your eyes!) to read the music, especially when sight-reading.

If you embark on a long practice session, take a break every 45 minutes to move around.  Roll your shoulders backwards and gently move your neck.  Do some chin-tucks (pull your chin back to make a double-chin!) to reverse any 'forward head posture' which is the cause of much neck and shoulder strain.

When playing, make sure your feet are planted firmly on the floor in front of you, and sit on the front portion of the stool.  Small children may need a box to put their feet on so that they are comfortable.  Sit slightly forward on your 'sitting bones', and this will help you keep a straight back.  This means you won't get any back pain, and your whole upper body will be free to move around, making all your piano playing easier.

It can be difficult to remember the right seating position to use at home, when the teacher is not there. Luckily, Dr. Teresa Dybvig's great website The Well Balanced Pianist includes photographs of how you can make simple adjustments to your seating position to help achieve the most comfortable playing position.   I would also advise any pianist to watch the videos of Dorothy Taubman's approach to technique and posture, available on this Youtube playlist.