Very effective upper body posture correction exercise.
Move your head slowly backwards,
Remain looking straight ahead,
Do not tuck your chin or tilt your head backwards,
Hold for a few seconds,
Repeat as often as you can, anytime, every day.
Fine tuning: Reach up with the back of your neck to make it longer. Imagine a string attached to the top of your head pulling it to the ceiling. Do retractions anytime you can, as often as possible.
See Robin Mckenzie’s book, Treat your Own Neck at OPTP.com
Neck Retraction - Stretch/ Exercise