Posture: there are 4 common meditation postures. Laying down, sitting on the floor, sitting on a chair, standing up.

Back: strait. Leaning forward when sitting, may invite the exchange of meditation with sleep and may also invite back pain.

Mouth: lips closed, small gap (rice grain) between the upper and the lower teeth,

Tong: tong touching lightly the roof of the mouth while resting on the lower front teeth.
The tong is a “barometer of stress”, vacuum in the mouth, is a sign of tension.

preferably with eyes close. When eyes are closed the focus turns inside and chances for distractions is reduced. It is much easier to use the imagination when eyes are closed.