We all know tummy time is important for strengthening those upper body and neck muscles, but it's also important to ensure a nicely shaped head. Some things you can do to help include:
Ensure baby is not constantly in a seat or on their backs when awake. They should of course ALWAYS be placed on their backs for sleep.
Give baby at least 5 minutes of tummy time, several times a day.
Position baby in different ways, so they will turn different ways to look at things they find interesting.
Vary position in crib for same reason above, or even move crib around the room.
Hold baby, so they are not resting their heads against anything.
If your baby's head constantly seems to fall into the same position, you might also want to consider Torticollis, a tightening of the neck muscle, limiting range of motion. The best way to view the head for symmetry is to be higher than baby and look down at the overall shape, or lying them down as in the picture. If your baby's head looks flat or generally mishapen, talk to your pediatrician.