One trick into getting the illusion to work better is to avoid putting it in a light that is above it. The shadows caused by the downward light (on the top of the face) makes it tougher to see the illusion. The more uniform the light (i.e. a front light), makes it easier to make your eye think the folds (in the head) go the other way.
Thanks again for pointing this out to us here at the HTF! It is a really amazing little gimmick. I have made a couple of them now. I have one sitting on top of my computer and it just constantly amazes me how well the illusion works!
Nice find!
(Oh, and if you have not done so gang, print it off and try it out...it really is amazing! Use the PDF version if you are having size issues.)
Here is another optical illusion. Not as spectacular as NickSo's but really creepy. And I PROMISE it is not one of those screaming scary head things. http://www.ebaumsworld.com/optical.html