this works for you. i bring one leg up to a bent position. the other leg is straight. you have to experiment with different things. i even put tree 2x3's running lengthwise under the materess and over the box spring so bed is more firm. also try with both legs bent.

I have to use about 10 pillows. I can't sleep on my back anymore either. I sleep kind of like pete mentioned. I am mostly on my stomach and partly my side with a pillow under my stomach, one leg straight out with pillow under that ankle, other leg is bent with pillows between my knees. It used to be most comfortable on my sides but it would aggravate my hip pain and I would wake up and rotate sides every 30mins. I can't sleep on my stomach flat cuz of the facet pain so I have to have one bent leg and one straight and 2 pillows under stomach.

wrap it around dip in your back, i found it helped me to sleep for a short time on my back, knees bent with pillow under them