....tangerine, tangerine.... o tangerine...
How about this: Don't concentrate on directing your eyes to the back of your head or anything else weird or difficult 
Instead, FEEL the tangerine in the back of your head. Now, see an image of you yourself in your mind's eye (like in image streaming). Turn the image so that you can see the back of the head of the other you. Pull the image closer in front of you, about 30 cm from your eyes and make the head of the other you about same size as it is in reality. And see the tangerine all the time. (You can even mell it!)
Now, combine these two: feeling the tangerine and seeing it on the back of the head of the other you. (An smelling ) It causes a bit lightheaded feeling first, but soon the Photofocus appears and you get a concentrated feeling.
I tested 5 minutes ago that this works, at least for me. Hope it works for you too! -P
