Kazzy01,
I have endured this psychometric testing nemerous times in Airservices. Their is nothing you can do to prepare for it other than lay off the turps and get a good night's sleep.
It seems to be an endless stream of questions along the lines of "On your day off, would you prefer to go to the zoo or to the museum", and by the time you've done a hundred of them that seem so similar, you'll be feeling loopy.
However, imbedded in the questions is a test for consistency, etc, that can detect if you start answering randomly.
If they offer you a debrief afterwards, take it. I was amazed at what they could determine about my personality! They say that it's not used so much for selection but rather to see the correlations of those who do become successful ATCers.
(Try a Google search.)