If you're physically able, you might check into prenatal yoga classes. I have a couple of healing herniated disks in my lower back, and I'm really worried about my back during the pregnancy so I just started prenatal yoga. My OB says the stretching and back strengthening it will give me will help a lot...you can tell the instructor about your back and she'll modify the exercises for you. I went to a physical therapist about a year ago for my back and she said the best way to avoid back pain is to strengthen the muscles that support your back.
Unfortunately, I have them too and have for awhile now. The only thing that helps me is to take a tylenol and put a pillow and heating pad behind me. (I put the heating pad inside the pillow case) The pillow supports my back and the heat soothes some of the pain. That, and trying not to do to much, which is hard when you have a 2 year old! I didn't have back pain with my last pregnancy until the last few weeks when I was huge! Anyway, hope this helps some:) I know it's no fun.