Pretty much always on my front, but sometimes on my side. Usually if it's very hot, or I've got a pain caused by sleeping on my front (which usually means I have hurt my toenails and if I'm on my front they are touching the bed and therefore hurt).
I think as a child I might have slept more on my side or back, but I remember being told by my cousin that if you slept on your front you wouldn't get nightmares, which I didnt often get but obviously found to be such a risk that I would take to sleeping on my front from then on.