Ah, y'see, to witness the back of your own head, you need a set-up rather like the Michaelson-Morley experiment, with two mirrors facing each other a suitably long distance apart. You put your head between the mirrors, looking stright at one of them, then you need to duck out of the way sideways, allowing the image of your scalp to pass by you after it's first reflection, turn your head through 180 degrees as the image does its 2nd bounce, and be ready to jump back in line as it does the third bounce, just in time to catch the image in your eye.

If you're not quite nimble enough for all this dodging & turning, then take another leaf out of M & M's book, and place your head on a rapidly rotating platform, with an oscillating arm that swings your head in and out of the light path in sync with the rotation.

Or then again, you could just *very slightly* angle the two mirrors away from each other and then sit and leisurelililililily admire your hindbrain after only 2 reflections, for as long as you like - but where's the fun in that? =:o}