Actually it is best to have a soft as a matress as possible. It is a common misconception that a firm matress is best but nowadays its beleived softer mattresses are better; i nearly bought a new mattress recently and spoke to alot of the staff at different bed shops. Obviously though it does sometime vary from person to person
I sleep on my stomach because if I try to sleep on my back I stay alert and don't drift off. If you can sleeping on your back is supposed to be the best, flollwed by your side (can't remember which side is best) and then your stomach. Apparently people live longer if they sleep on their back but in the book I read this in it didn't really back this point up.
According to who? Sleeping "experts" who are trying to get higher commissions by selling fancy soft mattresses?
The mattress is just the surface you sleep on, and that needs to be firm. As I said, though, it's only afaik. I find a soft mattress very uncomfortable, but I do have about 2 inches of memory foam as a cover, for support. I wouldn't call memory foam "soft", though.