Generally there are only two positions that are recommend for sleep.
The most recommended position is side sleeping
The way to measure the right height pillow is to lie on your side and to make sure the spine is in perfect alignment. The pillow must not cause the neck to tilt too much upwards or downwards and should allow for minimal pressure being exerted on the spine, neck and shoulder muscles.
The two primary factors to look at when choosing a pillow are:
1) The broadness of the shoulders (i.e. frame size of person)
2) The softness of the bed. If bed is very soft then shoulders will sink in and a lower pillow will be required. If bed is very firm then shoulders will not press into the bed so much and a higher pillow may be required to support the neck.
The second accepted position for sleeping is on the back
If purely a back sleeper then a lower pillow is the right way to go. You will just need something that supports the curvature of your neck in order to perfectly align the necks progressive curve. The softness of the mattress will play a slight part in deciding a back sleeping pillow but not as much as for side sleepers.
Back and Side Sleepers
If you are a back and side sleeper then we recommend finding a pillow that fits for side sleeping but is of a softer density so that when they roll onto their back the pillow will not cause undue pressure to the neck.
Many people do sleep on their tummy but the consensus is that this should be avoided as it will place unnecessary pressure on the lower back during rest. There will also be undue strain on the neck which may cause damage over the medium to longer term.
Some other hints
Other important factors to consider are if there are any other conditions either pre existing or preventative that need to be factored in. Also the length of time the user is in bed should be factored in when choosing a pillow as other supports may be required.
Most importantly whichever pillow you choose it must hold its shape during the night so as to give you the support that you are after for the whole night. For this it is recommended that pillows should have a good density (weight) to them to make sure they last the distance required.
[This information was supplied by our friends at Therapeutic Pillow Australia - we stock their products, and our Osteopaths can match you with the right pillow]