This is more long-term--- Shoes with good arch supports. I showed up at a midwife appt complaining of lower back pain and got a bit of a lecture cos I was wearing flip-flops. I *knew* flip-flops are bad - NO support whatsoever - but it's summer and they are so easy!!! So I had to chuck them out and wear my slip on clogs and other shoes that promote better posture.
I also do a number of yoga type stretches for my back pain- which is usually sciatica, different from yours, but these should help you too, I think. Angry Cat- on all fours (hands and knees) you move your lower back up towards the ceiling, so you are creating a kind of hump. Move from Cat to neutral and back again several times. Also pelvic tilts, like Melissa said. And pelvic circles on your hands and knees, as well as standing up, knees slightly bent, hands on hips.
I second (third? fourth?) the idea of finding some prenatal yoga classes too. Even though I am a mind-body-spirit practitioner of a different kind, I never much liked yoga until I tried pre-natal yoga. It's great! Let the teacher know what kind of issues you are having and she (likely a she) will almost surely have lots of ideas for you.
Wishing you vibes for less pain!