I'm only on my third right now, but plan on having 4, or maybe 5. I actually had the most problems with my first pregnancy. I wasn't in very good shape, and had more problems with ligament pain, flexibility, swelling, and moving in general.
I made a real effort to get in better shape. I have actually lost just a little bit of weight over each pregnancy (eating healthier and drinking more water). While pregnant and postpartum, doing prenatal yoga is a huge help. It is gentle, simple and easy on your body but helps keep you in shape. After my first one, maybe a couple of months later, I also picked up a postmaternity pilates DVD. I found that because it was postmaternity, it wasn't too hard. Actually, I could only do about a quarter of it or less at first, and I slowly built up to being able to do the whole thing. I'm not in the best shape so I find this postmaternity pilates to be challenging even when my body is in better shape.
I think the biggest thing that can help is doing something to build up those muscles between pregnancies and then maintaining them during the pregnancy.
And if you can afford it, I highly recommend going to accupuncture. I had my first two in Japan and it was recommended to me. At first I was skeptical but it makes a huge difference! It helped with back pain and ligament pain. Unfortunately, it's more expensive here and I can't afford it now.