The nose should be in the field of vision or the head has dropped forward. This serves to half close the eyes cutting out most of the visual input without providing the conditions to fall asleep. Place the tongue on the roof of the mouth and place the teeth lightly together. Draw the air out of the space between the tongue and the palate. This will cut down on saliva production and the need to swallow.
Breathing is done in a very specific fashion and is the most important aspect of the practice.
The ancient Taoists believed that breath was life and that each person was only allotted so many. Deep slow breathing was seen as life prolonging. Inhale easily and quietly through the nose using the diaphragm. The belly should expand forward while the chest should expand without any musculaular assistance at all. Keep all tension and muscular effort out of the upper body.