The other day I gave this advice to those who have difficulty looking at the ball: Don't look at it, look instead at a mark on the turf just behind (or just ahead) of it.
Now I want to take this advice a little further.
In my opinion one of the prime causes of the disorder called "head up" is optical. If you look away at one certain spot and then back again to what's in front of you, then when you look away again your eyes will usually pick out the exact spot you looked away to in the first instance. Try it now for me and see if I'm not right.
This means that if as we address the ball we look up several times at the target, perhaps the flag on the green or a specific area in the fairway, and then back at the ball, when we swing there's an automatic tendency to look up again at the exact spot we kept looking up at as we addressed the ball.
So here's another reason for looking behind the ball.
But in this case I advocate making a habit just before you start your backswing of looking briefly at a mark behind the ball along the line of the swing, two feet or so away, then either back at the ball or at the mark you are going to look at as you swing.
You will then convert the tendency of bringing the head up to look again at the target into a tendency to look back at the ball.
Try it. I think you will find I am right. Don't concentrate on doing it overmuch. Relax. Just make it a natural part of your habitual address routine.