Have a goal set (could be a uni course, dream atar etc.) and set your mind to achieve it. It'll motivate you to study during times of laziness, procrastination and whatnot.
If you are doing extra units e.g. 13, DO NOT WAIT AND DROP the subject you don't want to do very late into year 12. There is no purpose of doing an extra unit if you hate the subject (me with legal studies) as it simply wastes too much time = no motivation = poor marks. Choose the Subjects you want to do wisely and do not do something just because your friends are doing it. It is your HSC.