Method
Proper posture in meditation is important. The foremost concern is that the posture helps us to be both alert and relaxed at the same time.
Posture can help support a condition of alert, awake awareness by keeping the spine straight and the neck erect. Human (and probably all mammal) physiology reacts to a straight spine by intensifying wakefulness.
Posture can help support a relaxed, open state by allowing the rest of the muscle groups in the body (besides those directly involved in keeping the spine erect) to release and let go as much as possible.
Furthermore, it is also important to be as comfortable as possible. Perhaps the most comfortable posture would be lying down, but often that does not support wakefulness.
A final issue is that the posture should feel stable and balanced. There should be no sense of instability, a tendency to tip over, or a feeling of being off balance.
