I've just been invited to begin my ATC training with Eurocontrol (pending a medical) in March next year.
My advise to those who are going to phase 1 is to take a look at this link about the feast test. You can't really do any preperation because it is all about your natural ability and reactions. Just make sure you get a good nights sleep and are fully focussed.
FEAST test - Wikipedia, the free encyclopedia
Phase 2 - they look at you as a person rather than just test results. You will need to do a small presentation on a task they provide and see how well you work in a team in a group discussion. Again no preperation is needed. English will be assessed.
Phase 3 - Final Interview - Prepare for this like you would any other interview - research the company and the role.
Ollie