Biggest tip: Be honest. Generally the Psych interviews are quite long, and often they will ask you the same question a number of times during the 2 hours+ but in a different way to see if you give the same answer, or are just telling them what you think they want to hear.
Second biggest tip: Be honest.
Third biggest tip: Be honest.
For the Board, make sure you know WHY you are applying for your particular specialisation, where you can be posted in that specialisation, and as many specifics about that specialisation as possible. Ie, if you want to fly fast jets, find out where they are based, what a typical day consists of, what progression you can expect, what your future looks like after flying (in terms of promotion etc). Be able to tell the board about a Hornet and Hawk, their armament, how many we have etc. It all shows just how interested and motivated you are. You don;t need to be an expert, just know a bit about it.
If you get thrown some curly questions by the board (and you will), take your time, don;t get stressed and work through it. They want to see how you handle pressure etc.
Other than that, relax.
Finally, the biggest tip: Don;t lie.