Sleep supine. The best position for sleep is to be supine or laying on your back. This actually places the least amount of stress on your joints and prevents the development of arthritis, especially in your shoulders that many side sleepers eventually develop. The caveat to sleeping supine is that if one is a snorer then snoring, by virtue of gravity, has a tendency to be more prominent when laying flat.
Sleeping posture. You should lie down in a natural, comfortable position, with a comfortable pillow under your head to keep the correct head neck alignment, not straining any particular areas. If you are overweight or have sleep apnea, try lying on your side to prevent throat tissues from collapsing during sleep. If you have acid reflux, try elevating the head of bed. For back pain, use a pillow under your knees.