Check out Expecting Fitness
I forget the author's name, Bregetta something? (SOrry, I'm at work and don't have the book handy). Anyway, there's a thread somewhere on this forum with the name of the book -- the author was Jane Seymour's personal trainer and helped her get back in shape after her twins were born. She shows alternative exercises for pregnancy. (Dr. Clapp doesn't show any particular exercises, even though his book is great.) Mostly, this consists of working on the hands and knees while either contracting the abs into the spine or for obliques, doing "tail wags." I know Cathe described these moves in an earlier thread not too long ago, and that might be helpful (and cheaper than buying the book, unless you're still having trouble visualizing the moves).
As for laying on the back, you only have to stop that as you get bigger. It's the weight of the uterus and baby pressing on the big vein that returns blood from your legs to your heart that causes the problem -- too much pressure there can cause your (and consequently the baby's) blood pressure to drop. Some people say after the first trimester, some say after four months. I think I went a bit longer than that, but believe me, you'll know it when you get to the point where you can't exercise on your back anymore. You'll start feeling naueous and hot and then get really light-headed. That's when I knew it was time to stop.
Hope that helps somewhat!
Hazel