This is one of my favorite stretches:
At the bottom of an externally rotated toe-balance squat, with your heels just under your sit bones (1st position grande plié for the ballet speakers), grab onto your knees. Pull your knees outward as you press your knees into your hands. Lengthen your lower back then puff your chest and look upwards. This requires major ankle stability and a lot of upper back activation.
Photo by Pole Ninja Photography.