In the Alexander Technique, there is no “perfect posture” into which you should “sit up straight.” Instead, you learn how to use your sense of balance to stop holding too much tension or not enough tone. You learn to access your body’s own inner support structure. Postural balance includes toned muscles, aligned bones, lack of compression on joints, and ease of mobility. (Static sitting is sitting in stillness, fixed or collapsed in a position that is rigid or compressed. Most chairs are unfortunately designed for slumping into a static collapsed position.)
Chairs that promote active sitting (Balance Balls, Sissel pillows/sitting discs, or mobile stools) encourage movement and a dynamic postural balance.
But even on a static chair, you can balance upright.
Sense your sit bones on the chair and your feet flat on the floor.
Let your sit bones rest on the chair as you think of letting your skull go in the opposite direction, up to the ceiling. Allow your neck muscles to release into length, so that your head floats up and you allow your spine to follow.
If you can sense your spine moving upwards from the middle of you, rather than lifting yourself up from the front (chest/ribs) or forcing your back to be straight by holding muscles, then you are on the right track. Picture your spine, with it’s gentle curves. It is very wide and strong in the lower part, inside the pelvis, which is a bowl-shaped structure that can support you in balance on a chair. In the middle, the spine is supported by many tiny muscles deep within you. Visualize, imagine, or sense, that you can allow these deep small postural muscles to do their work, so that the larger muscles of your back do not have do any unnecessary holding. Closer to the top of the spine, the bones are smaller and more delicate. Can you sense that they and their surrounding tissues can be free of tension or collapse? Can you sense the delicacy of balance for your skull on top of your spine?
It does take time to develop these senses and this skill of balance, so practice in increments that feel reasonable to you.