I recently collected the following information which is IMHO damn important for proper sleep:

if you want to sleep better AND eliminate or at least improve back pain, sleep on a HARD mattress (as hard as possible) and on a pillow AS THIN as possible, or better, on NO PILLOW at all. To get there, slowly reduce the thickness of your pillow AND if you got a slatted frame, increase the hardness of the frame slowly. If you reach maximum slatted frame hardness and think you can still "sleep harder" ;-), get a mattress one grade harder, then repeat, start with the softest slatted frame setting and work your way towards harder. This will be uncomfortable in the beginning, but will pay off soon, for example, it will eliminate most cases of back pain and you will sleep better.
Sleep on your back or on your belly, try to avoid sideways. Turning is natural, your body wants to do it for various reasons, but try to get back into these two positions when you wake up etc.. You can train your body to stop sleeping sideways for the most part, might take some weeks though. All this is explained in a german video by "Liebscher & Bracht" ("the pain specialists") here:

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If you are smart, you combine this with inclining your bed see "Inclined Bed Therapy".