Good posture.  Maintain a straight spine at all times, which allows your pelvis to support the weight of your baby, rather than your back.

Sit in a supportive chair.  If your feet don't touch the ground, use a stool, which will allow your lower back to remain in alignment.

Sleep with a pillow between your legs, which will again allow your spine to remain in proper alignment.

Use a pillow.  While sitting on the couch, use a pillow to support your lower back.

Try tailor-sitting.  Tailor sitting, or sitting cross-legged on the floor, will help by releasing the muscles of the lower back.  Plus, it may have the added benefit of getting the baby in a good position for labor and provides a gentle stretch to the inner thighs, which will be beneficial when it comes time to birth your baby.

Try the pelvic rock.  If you've ever done yoga, you'll know this as a modified cat-cow stretch.  While on the floor, get on your hands and knees, making sure that your weight is evenly distributed and your back is flat like a table.  If you suffer from carpal-tunnel, make fists, rather than supporting your weight with a flat hand.  Now, arch your back up, like a cat.  Hold, and then return to the flat table position.  DO NOT drop your belly to the floor like a cow's back, as this will add additional strain to your back.  Repeat as often as needed.  Just like the tailor sit, this may help to get your baby in a good position for birth, and is an awesome position for labor.

Make a trip to your chiropractor.  A visit to the chiropractor might do the trick, as your pelvis acts like a hammock.  If the hammock is a little lopsided, back pain can result.  A visit to the chiropractor while pregnant rarely involves cracking, but rather gentle manipulation to brings things into alignment.  As always, check with your OB or midwife to get the OK first.  

Get a massage.  Tell dear husband that it really needs to be done weekly.  OK, maybe he won't buy it, but it is worth a try, right?