I was constantly rolling over and sleeping on my stomach.
I couldn't help it--it's how I slept before pregnancy.
When my belly was real big, I found myself STILL waking up on my stomach, though I was laying on the side of my stomach if that makes any sense.
I was always told that it is fine to sleep on your back until you are no longer comfortable.
It felt like sleeping with a watermelon on top of me.
A body pillow should work though. They are the extra long ones.