Keep your crosshair high and try to guess where the head is going to be. I used to see some players on pubs that have there crosshair at chest height and then during the firefight try to aim for the head. This isn't good.
If I am getting less headshots than kills than I am having a very bad game.
to get head shots i sometimes find that by opening up on the chest then moving up to the head can be useful. you're more likely to hit the chest, stunning them then move up to the head to finish them off. :wink: