There is info already on here about this, but ... well .... ok.
1) Psychometric testing
Checks your brain is working properly. There are books on the topic - I looked at several but none of them were like the actual tests.
Since all the tests have a visual element and some an audio element, it is difficult to explain in words what they are/were like.
2) Psychologist interview
I thought this was quite a nice experience, some of the others who did it at the same time didn't enjoy it much.... Many thought the questions asked were too personal - I didn't mind them at all. I guess it depends on what kind of person you are.
In the end though, no one failed on the basis of this interview when I was doing it.
3) Simulator Test
Who knows when this part comes! Some insist on an IR first, others on a JAA IR while some send people with no IR to do it.
Anyway, they get you to fly around on instruments and do an IR approach and auto. There is a second crew-member so you have some help on standby if you need it.
Quite good fun actually - if there wasn't a job riding on it
Overall the test process isn't really that hard if you're at least averagely intelligent and emotionally stable. For those that aren't, no amount of prep is going to help and I wouldn't want them in the aircraft with me!
When I did it, everyone else seemed to know more about what was coming test wise than I did, but that didn't seem to help them much...
So I would say the best prep is to relax and be yourself, maybe do a few mental agility puzzles from the newspapers/mags just to get your brain working and enjoy the process.
BG.