If you play the game enough, you'll get this on accident, but there are ways to make headshots easier. At the beginning of levels where a sniper rifle is available in the saferoom, such as Death Toll 4, take it and pop as many zombie heads as you can before leaving the safe area.
The quickest way I found, though, is to grab a pipe bomb and an uzi or assault rifle. Throw the pipe bomb, and shoot at the clump of zombies that forms around it at head level. You'll get a bunch of quick headshots that way.