Inihbit or (stretch if not available) :

Pec-major and minor. This could be pulling the shoulders forward
Upper trapezius- Lifting the scapula up
Levator Scapulae-Lifting the scapula up
Sternocleidomastoid- Locking the head on the neck
Scalenes- Pulling the head forward and rounding the back
Subscapularis- affecting the shouder blades in having round shoulders
Lats- Short, tight Lats could be rounding the back and bringing the scapula in downward rotation
Mobility of the spine in all planes especially extension
Work on mobility of the scapula such as arm circles
relax shoulder

Activate or (strengten if not available)

back extensors of the neck & back
MIddle & lower trapesius- helps with shoulder stability and placement
Anterior serratus- helps with shoulder stability and placement
Rhomboid- helps with shoulder stability and placement
Rotator cuff- helps with shoulder stability and placement
How are legs affecting posture
Strengthen core especially in relationship to neutral spine and extension


pendulum stretching exercises.

http://www.youtube.com/watch?v=jcsefD4FuzI

http://www.exrx.net/ExInfo/Posture.html

this site is great

not that i advocate lifting weights but if gives you an idea