Sitting down
Sitting down from a standing posture is a logical exercise for those who took the time to think about it. In standing, you are in contact with the ground in only 2 points (the feet). In sitting, you contact the ground (via the chair) via 2 more, the 2 sitting bones. Therefore, you need to move from a position with 2 points of contact to a position with 4 points of contact.
Hence, your buttocks need to move backward. Remember the rules governing the physiology of the human body: if something moves backward, something else needs to move forward. In this case, you have to bend forward in order to let your buttocks go backward. This automatically brings you closer to the chair. Before that, you should have kept your neck flexed to protect it (hence also reducing the tension in the low back). Once you are in contact with the chair, the normal raising procedure applies: use your hips as a crane to bring your upper body upward and bring your thoracic vertebrae slightly forward when your hips cannot turn anymore. This is illustrated below as well as in this video: flv /wmv.
The picture below shows why the chin needs to be kept in the chest before bending forward.
