The most important thing in taking the zazen posture is to keep your spine straight. Your ears and your shoulders should be on one line. Relax your shoulders, and push up towards the ceiling with the back of your head. And you should pull your chin in. When your chin is tilted up, you have no strength in your posture; you are probably dreaming. Also to gain strength in your posture, press your diaphragm down towards your hara, or lower abdomen.
