The correct standing posture (chest out) includes the following requirements.  Mind you, it might be different for each person: 
A. Push the back of your head back, and retract your lower jaw slightly, so that the curvature of your cervical vertebra can be restored. 
B. The scapula naturally retracts.  For some people, due to round shoulders, the trapezius muscles are not balanced, and scapula needs to be lowered, so you should exert yourself backwards and downwards.
C. Back and stomach exert force alternately.