By encouraging freedom over the back and through the shoulders. An inappropriate saddle or needing a chiropractic adjustment or muscle therapy can prevent freedom.
Also riding, can help to put more weight in the stirrups to take weight off of the back. The goal being to sit as lightly but still be deep as possible. Also by following the movement of the horse's rib cage when the left rib cage pushes into your right leg follow that movement with your right leg and allow your legs to rock the barrel of the horse and allow swing and movement over the back. Free walk is very good for this. As well as gathering up having a nice soft reaction and then lengthen the reins and neck, allow swing and gather again. the accordion like way of riding can help loosen a tense horse.