Calleigh used to favour the one side to lie on. And she didnt seem as though she could turn top the other side as good.
It did start to effect the shape of her head and we started with repostioning in which i would turn her head the other way in her sleep. That didnt work so well as she would turn it back within minutes at times.
Place toys to the side she doesnt lie on to encourage her to turn that way.
Plenty of tummy time is advised also. Calleigh didnt like tummy time and still isnt a great fan
There is a condition called Torticollis which is to do with the muscles in the neck.
Gem&Leland on the forum will probably be able to advise you better, she knows alot about it due to Leland having it. She gave me some great advice when i was worried about Calleigh. I am sure she wont mind if you ask her
Also she has a thread about Lelands Plagio treatment you might find helpful to read.
Then one day Calleigh just suddenly started to use her other side which shocked us as it had gone from not using at all to using it as much as the other side. Her head looks alot better now also. So the repositioning did pay off in the end even though i didnt feel as though it was making any difference.