Sometimes I still sleep on my tummy, but not often, because of those annoying things called breasts! Still, when I do this I sleep very well and don't get back pain.
When I sleep on my tummy, I face right, left arm bent with my forehead resting on my forearm, right arm bent with the hand closed by my chin.
With the arms like that, I have room for my breasts to hang out without being squished by the bed.
My right leg is bent slightly, and my left leg relaxed.
It's wonderful :-)
This aligns my hips/shoulders, and my waist doesn't touch the bed (that is, it doesn't slump).
As I kid I slept on my tummy, but I saw my mom sleeping in that position. I would ask her all the time, "Mom, doesn't your arm fall asleep like that?" and she'd reply "No, of-course not!"
Now that I'm a grown woman that position seems to work for me too.
Apparently that is the most common position women use when co-sleeping with a baby, for easy breastfeeding, so it makes sense. Like a cat with her kittens.
As for grown men... I don't know, I'm not one. But I've never seen a man (brothers, cousins, friends, boyfriends, father) sleep in another position other than on his back :-P