There is no fool proof way to get a good mark in the HSC. To get a band six, I'd suggest you put a social life on hold for the next 6 months and just study, study, study.
Infact, the best preparation you can do for exams now is past HSC papers.
I just did my half yearlies
Biology: Know the syllabus. Sciences are easy when it comes to the Syllabus, because often the syllabus note is similar in wording to questions in exams. Have a few experiments you've done handy, as well as examples of animals that display similar traits (for example, have an animal that is ectotherm, endotherm, examples of enantiostasis, etc), to back up information.
History, you didn't specify which history. With Ancient you just need to know basics, dates are handy. If you're studying a personality, make sure you know their family background, political background, their actual personality, the way they interacted with their people and their influence on whichever civilisation they ruled or were part of.
If it's an ancient society, know customs (burial customs, funery customs), know evidence like frescos, know trade partners, things like that.
English Advanced is the easiest. Know your journey's booklet, know Skrzynecki, know King Lear productions and KNOW THE PLAY! Don't worry about interpretations too much, one or two for King Lear is enough. Make sure you have quotes to back everything up, three or four quotes will get you safely through. The same with Skrzynecki, quotes!!!
Extension 1 English - Know your texts like the back of your hand. Be able to relate your texts to your module, whatever it happens to be. Quotes, examples. Be ably to relay the texts like you know them word for word. When it comes to creative writing in ext, be able to make your creative task relate to your module.
Easy as.