As Dr S says .... be yourself. Don't listen to the braggards either - none of them got through selection on my board (many moons ago). Listen to what's being asked of you in group exercises and look before you leap. Try not to dismiss other people's suggestions out of hand - at least pretend to consider them before canning them!
As far as aptitude tests go, brush up on mental arithmetic, but don't try to improve your co-ordination by playing computer games .... you'd be better off playing a sport for hand/eye co-ordination.
Be positive in the interview and be prepared for Q's like "Why do you want to join?", "If you are unsuccessful in being selected for your preferred role, would you consider....?" "Why are we in Sierra Leone?" "How many GR4 Squadrons do we have, and where are they based/deployed?" "Why were there riots recently in Genoa?"
As for interview technique, I'm no expert, but try not to fidget, sit up straight and maintain good eye contact throughout.
Hope this helps.....and good luck!!