My last little one was posterior throughout the entire labor. I was very active and sat on an exercise ball through most of my labor. When my water broke on it's own she rotated all on her own and I felt her head go 180 degrees against my cervix - pretty cool - I was standing at the time. I am convinced that if I had been laying down through labor and if my waters had been ruptured artifically she would not have rotated. I think being in a hosptial where you're laying down and AROM usualy happens you are more likely to deliver posterior.
I have heard what to do with a posterior baby in labor. This is from a childbirth educator who read a booklet on it written by a woman who had 4 posterior babies. I have not tried this myself, but apparently, it works really well. What you do is, every time you have a contraction, you clasp your hands together below your belly and pull them upwards. You may want oil or lotion on your belly. This directs the energy of the contraction upwards so that the baby goes downwards. According to this booklet, this woman's babies came very quickly because of what she was doing. HTH.