You may just need to sleep with more support under your back (like an extra pillow or rolled towel or blanket). I've suffered with chronic back pain for five or six years now, so I'm used to it and my back pain rarely seems out the ordinary enough to attribute it to my pregnancy. But hopefully I can offer some help!
It could be your mattress though. If you have a pillowtop or something, that could be an issue too. When my sister was pregnant with her first, she made my BIL buy a new mattress because she was so uncomfortable because their bed was too soft. I have to sleep on a rock hard mattress too or I won't be able to walk the next day.
I find that doing some strength training exercises that target the upper, middle, and low back helps me. I was having a lot of upper back pain when I stopped working out for a few weeks and now that I'm getting back into it, my pain has gone down. I also spend quite a bit of time sitting on a stability ball, which helps with core strength and balance, and that's helped too. You could also try some yoga poses. I have a prenatal yoga book that was a gift from my husband's aunt that I haven't looked at yet but I'm excited to try the dvd that came with it.
Re: Back Pain Anyone
I could have sworn I had pain the other day, from my old epidural 7 years ago :-??