One of my favorite back stretches is to lie on my back with my arms out to the sides, then lean your bent legs over to one side, and turn your neck to the other. A twist. Try to keep both shoulders level on the ground (one will try to lift). Go gently
If you're very tight, put a cushion under the knees to support them in a higher position. Lay there a good long time on each side. i find a give a little more every 10 breaths or so.