You never know what you're going to be asked, but I prepared for my interview by learning about the CTC partner airlines (history of each one and fleet information), and also reading up about the aviation industry in general, e.g. what are the current "hot topics".
The level of technical questions I think depends on your experience; the ATP guys on my Stage 3 got asked much more technical stuff, where as my interview (cadet) was about teamwork, leadership etc. as I have very little flying experience.
Apart from that there isn't much you can 'prepare' for. You don't want to sound too parrot-like. Have a few examples ready to back up what you want to say, know about your subject and be yourself. You will inevitably get a question you hadn't thought of, and how you cope with that will be important.
Good luck!