A large amount of email that we respond to deals with sleep position and trying to find the most comfortable and supportive pillow for that position. Side sleepers especially need support and a proper position to avoid neck pain.
Most studies show that side sleeping with slightly bent legs and using a body pillow between the knees is excellent for proper spinal alignment. For your head using a puffy synthetic or larger fill down/feather pillow will ensure your head and neck stay in proper alignment.
For people that prefer down and softer pillows using two will allow for proper support and comfort.
The key to side sleeping is remembering to keep the spine, legs, and head all on a parallel plane to ensure no pain come morning.