There are many definitions of good posture. Some of the best come from Martial Arts. In most Asian hand fighting disciplines the best posture for standing is as follows:
feet firmly on the ground a shoulder width apart, parallel to each other
knees lightly bent; they must never be locked
chest lightly expanded forward and buttocks lightly extended backwards to enhance spinal curvature
hands either by sides or bent and held forward in loose fist clench
remain relaxed but able to move easily
Our posture, or the way we hold our body, is fundamental to how well we function as human beings.