Two simple rules: 

• Don’t sleep on your stomach
• Try to keep a neutral, centered position whether on your side or back.

Sleep on your stomach requires you to keep your head turned to the side in order to properly breathe. This results in your neck being placed into significant rotation for an extended period of time. You can imagine the stiff neck you’d have if you sat in a chair and turned your head to the right for 6 hours. 

Keeping your body in a neutral position is crucial for the health of your sacro-iliac joints.  Swinging your leg out to the side or across your midline can physically stress or shear the S-I joint. Placing a pillow between the knees (side sleepers) or under your knees (back sleepers) will often help keep a nice, safe position once you asleep.

Mattress quality is more of a subjective issue. Finding the right balance between firm and comfortable is the goal.