Hi there - there have been some great posts here and I won't repeat what others have said, agree with onwardsandupwards - I had two OP babies, one was born stargazing also.
What helped me in both labours was lots of sacral pressure i.e having firm constant pressure applied to the sacrum i.e the lower back just above the crack of the bum! A heat pack really helped as well. I didn't have the opportunity of using a pool with my first labour but had a waterbirth with my second and it was lovely, the warm water really helped me relax and took away a lot of the back pain. I have never used a TENS nmachine myself but when I was a doula lots of my clients used them and they really seemed to help with a back labour.
There is a school of thought that OP babies tend to rotate during labour - if the mother is allowed to be mobile and use positioning etc to help the baby turn with the contractions. Epidurals etc tend to interfere with this process which might be one reason why you see so many ventouse etc needed with epidurals.
One thing to be aware of is that this process might take a long time and until the baby has turned then not much cervical dilatation will be going on - this only happens once the baby has rotated and the head is nicely applied against the cervix (all of this is less important with a second baby by the way). So it's really important to be patient and let labour take it's course especially during the early part - staying at home until you are sure that labour is really well and truly underway can really help and that's where a good doula can be really worth her salt. Resting a lot and making sure you get enough to eat and drink is also really important (another good reason to stay at home)
Anyway the important thing is that babies tend to rotate round from in a clockwise direction i.e around the mothers back not around her tummy - which is why you get such a lot of back pain as the baby's head is pushing against your sacrum. They are trying to get themselves into the LOA position which is the optimum position for birth. Physiologically it's difficult for them to rotate the other way.
Funnily enough what helps the baby turn is not necessarily getting onto all fours (although this will help relieve the back pain) but lying on your side - first the right side, then the left. This called Rotational Positioning and it's a little bit different than the Spinning Babies approach which tends to emphasise trying to encourage babies to turn before labour. Is your doula training with Childbirth International? If so then she will know all about this technique.
It might all sound like a whole load of gobbledigook but I swear it works. Sorry if this all sounds a bit preachy but it's something I'm a bit passionate about having had two OP babies myself!
Good luck - am sure you'll be fine!