Because humans spend the better part of their lives asleep, a position in which the back is lined up and not stretched too far in any one direction is a healthy start. In general, choose a firm sleeping surface to prevent your spine from "sagging" due to the softness. Try using pillows to support your knees and legs when possible. For instance, use a pillow between your legs when you turn onto your side. This helps align the spine and takes pressure off the side of the hip. When lying on your back, consider putting a pillow under your knees to ease pressure on your spine. Be sure to use a pillow to support your head and neck, too.