How can a parent prevent infant skull "deformity"?

Change head position. The best prevention, and treatment for infants under 3 months of age, is not allowing the baby to sleep with the same head position. One the baby is fully asleep, change the head position to the opposite side of the previous sleep period. If there already is head "deformity", have the infant sleep on the normal or less deformed side consistently until the head becomes normal appearing.
Many ways. One, is minding the plasticity and frailty of the young bones (consistence is very soft) and alternating the sides when feeding the baby (eg not always holding the bottle with the same hand, or the baby), the side the baby sleeps, etc.