Ditto Autofly,
I started passing aptitude tests when I started relaxing in them and taking them in my stride rather then getting myself worked up too much. This comes back to preparation. You need to put in the practise so you can be as confident and relaxed as poss. when it comes to the real thing. It also comes down to knowing what to expect and the more you keep doing the tests, the more relaxed and confident you'll become.
Stick at it!!
Good luck!!