Hey Belle,
there are plenty of sites (including this one if I remember rightly) on optimal foetal positioning. Generally, they say that the baby's back is the heaviest, so anything to do with gravity that can make your baby's back come forward will help. Apparently breast stroke is good (without the normal stroke legs, just do a kick), as well as rocking on all fours, sitting with your legs lower than your bottom, sleeping on your tummy (with lots of pillows). According to some articles, it is not enough to do it for a few minutes, you have to do it for an hour or so.

As for the back pain, I am not sure. My SIL had terrible back pain when bubs was posterior, but that was in labour, it is supposed to give you a back labour. Not sure about pre-labour though, that could just be standard pregnancy back ache!

Good luck!