There is no secret, just be yourself. If you try to 'be' what you think the RAAF is looking for you will disappoint and be disappointed. (or not appointed to be more correct) However the ability to count backwards from 100 by 7 could come in handy.
What to wear? Take an educated guess, its hard to be overdressed, v. bad to be underdressed. Finally, its best to remain clothed at all times, even after 67 pints of lager.