The best pieces of advice I have are...
*DON'T STRESS OUT.. There is live after the HSC (for the 1000th time)
*If you don't feel like studying then don't. You'll just waste time and get nothing out of it. Go for a walk, have a nap/sleep, go to bed early, do some exercise, watch TV, read a book etc..
*As everyone has said, DO MATHS HOMEWORK, espically for extension.. Past papers/questions are they key for maths
*I had a job for year 12, as much as everyone likes to tell you not to have a job I found it really good. (I only work 8-10 hours a week) Besides the money, my work is a very social and it was/still is a good release.. That being said, if you have a job doing night fill or something like that then maybe you should try working during the afternoon or on the weekend
*Do your assessment tasks as soon as possible. A girl in my year did all her assessment tasks the day before they were due and slept for the next couple of days at school after.. Not Recommended
*Listen in class. Again I know it's been said a thousand times but if you listen in class and do your homework your halfway there
*Get heaps of feedback from your teachers and make sure they go through your half yearly and trial exams so you know where to improve
Hope that helps