Correct posture is of vital importance to the performer. The singer in rehearsal sits at the end of the chair with the shoulders back and the torso lifted up out of the hips. This frees the diaphram to expand downward from below the rib cage. Also, lifting the head and neck out of the rib cage allows free air passage.
Use these suggestions also when standing. In addition, do not lock the knees as this will cut off circulation from the legs. Rather have a slight bend in the knees, tucking the derriere under the body. This will help to ensure proper breathing and freedom in breathing.