|Supervise tummy time when Baby's awake!|
It's important for babies to sleep on their back, but they tend to have their head facing one direction or another. They should alternate which side they face, but many babies have a stiff neck and favor looking to one side. Think of when you wake with a stiff neck - probably from positioning overnight. Many babies are in the same position for quite awhile at the end of pregnancy - of course they're stiff!
If Baby's neck is stiff, you can massage his or her neck and shoulder muscles gently and then slowly move the head right and left (chin to each shoulder) and side to side (ear to shoulder). Don't quickly force the head movement, but think of what you do if you have a stiff and sore neck. The more frequently you stretch it out, the better it feels, right? I recommend stretching Baby's neck with each diaper change (before the change or after you wash your hands!) until it isn't stiff for several days and Baby moves his or her head easily without your help.
Start supervised tummy time early on - the longer you wait to start, the more Baby might resist it. I see so many parents who are hesitant to put Baby on his or her stomach. Concerns range from the umbilical cord stump still being on and bothering the baby (it won't) to spitting up will worsen (test it out, for many babies it's actually better) to "I thought babies should never be on their stomachs" (only when sleeping or not supervised).
Tummy time is an important time for baby to develop muscle strength. It needs to be supervised, but it can be a fun time to interact with Baby. Lay face to face and talk to Baby, encouraging him or her to look up. Grab a brightly colored object and move it around for Baby to watch. Enjoy your play time.
For more information on issues related to stiff necks in babies and how to treat the stiffness, see this Torticollis information.
AAP article on how to prevent flat heads in babies.