How long do you have to prepare, and is it for a scholarship, or Direct Entry?
You are not supposed to be able to prepare for the aptitude tests, and in reality if its your first time sitting them (you are allowed to twice) then you cant really, at least for the hand-eye coordination and multi-tasking. Just read the intructions and take your time. I would spend a fair amount of time on mental arithmetic, mental reasoning and IQ style tests - plenty of good books in WHSmith!
The thing you can do is be up to speed for after the aptitude tests (assuming you pass)
Know the RAF inside out.
What is stationed where, and why?
What new a/c are coming? (not just the eurofighter!)
If you were to be selected, where would you go, and at what stage?
Why do you want to be a pilot?
Why do you want to be in the RAF?
They want you to be an officer first, pilot second remember.
Don't give the usual 'since I first saw an aeroplane' or 'to be the best' chuff - really think about it, because they WILL ask you.
Know yourself inside out aswell - take past experiences and tell them what they taught you, and the transferable skills you gained - leadership, teamwork etc.
Be fit aswell - if you can score an 'excellent' in the fitness test (I think its over level 10.6 in the bleep test) then its a big tick in the right box.
Good luck mate, if you keep a level head, stay focussed and portray yourself in a positive way you will learn alot from the experience, whatever the outcome!
JP