Don't think in your head, try to map out all your thinking processes and reasoning on paper even if they aren't going to be part of the paper. Draw rough diagrams on paper. And don't do the questions in order, last year I missed out on one of the easier questions (Q4, 5, or 6? the one to do with functions) because I did them chronologically. If you get stuck on a question, just jump to the next one. Also remember that this competition is the complete opposite to VCE Maths, it's about presenting your solution or logic in a clearly understandable way rather than getting the answer. That can be quite hard, I remember for the chessboard question it took me 2-3 pages of reasoning for me to be satisfied with my answer >.<.