I have tried to figure out ways to stretch it out myself. The best I came up with is these two. One is a fairly common stretch and doesnt quite seem to hit it perfectly. Sit on the ground legs out in front. Pull up left knee and place left foot to the right of the out stretched right leg. Brace your right elbow against your bent left leg, take a few deep breaths, breathe out completely, and twist slowly..If you cannot hold your breath long enough, fequent very short breaths will help. Switch sides. A lung full of air seems to prevent the full stretch.
The other one I found was when I was warming up for the deadlift. Bend over with a bent back and grab the bar with one hand. Angle your stance (stretch side foot farther away) away from the side you wish to stretch away from the bar, lean away/stand up, and side bend away from stetching side. Its kind of wierd to get the positioning right, but you will know it when you get it. It seems to hit it a lot better than any other back stretch i have heard of. Hopefully that helps.