http://www.babycentre.co.uk/pregnancy/fitn...gbackandpelvis/
http://meriter.staywellsolutionsonline.com...neck/pregnancy/
http://www.spine-health.com/wellness/exerc...uring-pregnancy
A lot of the info in the links is the same, but it is useful. The BEST way to prevent back pain is by keeping mobile. Even when your back hurts, actually especially when your back hurts. By staying sedentary, laying around or sitting for long periods you cause the muscle to tighten up, which increases spasm. Also not moving weakens your muscles, and you need them to stabilize your pelvis and back, especially while pregnant since the hormones are really causing things to loosen up.
I try to do what we call 'abdominal bracing' all the time. You would probably think of it as sucking in your gut. You want to make sure you get the muscle at the very bottom to fire, this is the transverse abdominus, it supports the pelvis. If you put your thumbs on your belly button and your fingers downward towards your pubic bone and then laugh or cough, you will feel that muscle contract. That is the one to focus on. Hold it in while you walk, lift, climb stairs, sit at a traffic light. It will help you maintain or build strength while you are doing everyday activities. I tell this to my patients all the time, and many of them that had long term back pain start to feel better after a few weeks.
The blinkie in my siggy has the exact hand positioning I described.