Our paed said our DS was favouring his right side fairly early on (he had bad jaundice which lasted 3 months and was seeing a paed when he was quite young). He told us to encourage Lincoln to turn to the left as much as possible and to hold him more on that side when putting him to sleep. It seemed to work as now he has no favourite side and we've avoided a flat spot. My sister (who's a midwife) said it can correct itself later on though.