Oh I was looking for Sharon Butlers "tape trick" the other day.

Basically you have someone help you get into proper shoulder alignment and they place a large strip of medical type tape from upper back over the shoulder to the chest.
approx where the bra strap would be.

then throughout the day if you start to roll or slouch the shoulders the tape will tug and remind you to stay/get into correct alignment.

If you are in high pain or quite out of alignment start with tape in a better than your usual ,but less than the perfect alignment - and work your way up to the best.