I have had pretty bad posture that I am working on, too. You might reconsider the exercise, as part of the problem becomes a lack of strength in the muscles that hold you erect, so if you try to correct it without strengthening those muscles, you may just find yourself in more pain from taxing weak muscles. A combination of strength training and yoga would probably benefit you a lot! Also, I found this band that holds your knees with loops and runs behind your back (from one knee to the other) that can be tightened to give some lower back support and promote good posture. I use it when I sew. I can't remember the name, but I'll post back here with it when I get out to my sewing room.