Go and see your Training and Development Flight or FDS or whatever they are called these days, they should sort out interview techniques and have loads of good gen for you.
Speak to your JEngO, he should point you in the right direction as a starter. Also might be worth contacting a base which does the type of flying you want to end up doing if you aren't already at one and see if you can have a visit - get it from the horses mouth etc.
The best practice for Aptitude tests is to do loads of Speed/Distance/Time type stuff, there are probably some good books out there that people can suggest?
Good luck
DS