As watatank says, "Practice".
Practise, practise, practise (that's the verb now) and keep practising.
At this stage you should probably be focussing on past exam papers. Try to get as many as papers as you can from other schools or external sources. Save up the actual HSC papers until a couple of days before the exam.
As you do more and more practice exams you get a little bit better with using time in the exam more efficiently...
... then ...
In the exam, just take your time. Be cool, be calm, be confident. Read each question carefully and make sure you answer the question. Before you move on to the next question or the next booklet, just check your answer.
Sounds easy, hey!