The basics of proper sleep posture are fairly simple: do not ever lay on your belly, on your back is best. When on your belly you must turn your head to breathe and this causes unecessary torsion forces to your upper neck. If you could use a small roll type pillow that fits in the crook of the back of your neck just below your head and sleep on your back you will be helping yourself dramatically. This posture can aid in better nerve and blood flow up to the head and down to the body. Using a full sized pillow or two to put under your knees will usually alleviate any low back pain as well. Ideally, you will sleep each and every night for 6-10 hours, this is by far a most important area to focus on in terms of posture. If you know what to do and what not to do while you are going to sleep there is a better chance of you being in that position or close to it while you do sleep.
Sleeping on your side is not as good as on your back but is better than on your belly. This seems to be the most common posture for most, but it is difficult to know if your pillow or pillows are the right size for your neck while on your side.
People tend to favor a side to sleep on and the weight bearing shoulder becomes compressed restricting blood and nerve flow which may cause pain or other signs of mal-function in the body.
Another quick tip for you is that clothing may also restrict blood flow, so try to wear loosely fitting clothing or no clothing at all when possible.
Lastly, another position to avoid is lying on the couch with your head propped up on the armrest, this can kink the neck and cause the muscle tone to become imbalanced.
It's mainly about symmetry, alignment, proper nerve function and homeostasis.
Life is good when you are in proper alignment and functioning well, improving your sleep posture is a sure way to help yourself function better and reap the rewards.