Proper positioning. Since 1992, when the usa started putting babies to sleep on their backs, there has been an increase in temporary flattening of the head (plagiocephaly). This resolves with time and may take years. Helmets are rarely recommended as the don't provide much benefit over waiting. Limiting time on the back when awake is most important.
Re-position the baby. A baby's head may become flattened on one side or in the back because of the way the head is positioned when lying down. Encourage the baby to turn the head from one side to the other by showing toys and having baby track with the eyes, or by using playful noises that attract his/her attention to one side then the other.