Actually, the side sleeper position is a good position for your back and neck. As with most things, there are variations that can make it an even better choice. If you are having trouble with body pain or stiffness, try some of these modifications.
A pillow for a side sleeper will need to be thicker than for a back sleeper. The reason for this is that you want to fill the space between your neck and the point of your shoulder. If you don't, your neck will hang down to the side, causing over stretching on the up side and shortening on the down side.
Another way you might compensate for a pillow that is too small, is to roll your down shoulder underneath, twisting your upper back or put your arm up under your head stressing your shoulder.
You don't want the pillow to be too thick either, because then your head is pushed up and over stretched on the down side. I find when you are using too much pillow, you may be craving more support under your neck. In this case, consider getting a cervical pillow. Alternatively, try putting a small travel pillow or rolled/folded up towel under your regular pillow pulled down to right over your shoulder. This will create more support under your neck, while allowing your head to settle into the pillow and not be pushed up excessively.
I will confess, I use four pillows... but there is perfectly logical explanation for that =)
If you have lower back pain or stiffness in the morning, try adding a pillow between your knees. This helps to level the pelvis, keeping it from tilting and twisting during the night. These motions stress the sacroiliac joints (between the hip bones and the triangular bone at the bottom of the spine) and the junction between the spine and your sacrum (triangular bone that supports your spine), and well as your lumbar spine.
If you have a narrow waist and wider hips and shoulders, or are pregnant, try a small pillow under your waist to support your low back.
Another thing that might be helpful, is a rolled up towel or body pillow behind your back to support you, or a pillow to hug to support your shoulders and keep you from rolling onto your stomach.
Do you have a bed that's too soft, and you can't replace it. Maybe you have an old bed that doesn't support you, or you just got a new bed that is too soft, or have a partner that likes the bed softer that you do.
You can actually make your bed more firm by putting a sheet of plywood between the mattress and the box spring. You can either do it under the whole bed, or just one half. I have tried this and found it quite comfortable.
Even though this sounds so comfy, your back doesn't think so! When you stay in this position for long periods of time, it shortens the muscles in the front of the body, and lengthens the ones in the back. Most of us are already doing this all day long with sitting, slouching, and doing work in front of us. So it is a really bad idea to do it all night too. Instead, make a conscious effort to straighten your back and legs as much as possible, especially at the hip joints. If you tend to curl up, try a body pillow to keep you straighter.