Hi Newbert and others.
Some points that can be considered very important and have proven themselves during my teaching experience. Also the principles of Alexander's technique and different lecture confirm it.
1) The basis for a free and efficient sitting posture is the natural support and position of your backbone. The frank saying to sit as "straight" as possible does not fully overlap with the best position for backbone. Any constructed force and uncomfortable habits should be avoided.
The best way is a well balanced backbone. If the parts of your backbone are well balanced, it prevents a lot of muscles in your body from working and trying to hold you "straight". If you are too straight it starts to cost you energy and create tensions. Also feel how your head is balanced on the top of the backbone and feel the line from sittingbones till your head.
2) How to achieve it?
The sitting bones should support the weight of the body. Feel how you connect to the surface you sit on through your sitting bones.
You should sit on the edge of the stool - there is a very simple reason why this is good. If you sit too much backwards, part of the support is given by hamstrings and the balance of backbone is lost and muscles start to force the "right" position again. Try it out and you immediately feel the difference.
3) How high or low should you sit?
If you are in the balanced position, you should choose a height where your elbows are at the same height with keys.
4) How far should you sit from the piano?
Your elbows should also be able to move in front of your torso and you should also be able to play the highest keys with your left hand and lowest keys with your right hand without any "dramatic deformations" of you sitting posture.
If there are any more questions I would glady give my best to answer you
Best wishes to you!