So Yr 12 is just started for all you new Yr 12's. I suppose i have a few top tips.
1- actually do your assessments, and on time, and well, i know alot of people, and me who did get dragged down by their assessment marks, although i was fortuneate it wasnt THAT much, but its still alot. So try you're best, as annoying as assessments are.
2- Keep up with homework, and pracs (for the sciences esp. i found making sure Bio was all done was the best idea, and that my Bio Teacher marked everysingle one, so my class did ALL of them [over 100] where some schools didnt [ie. my ex's] so he ended up using some of mine)
3- Pay attention, sounds simple enough but it helps more than you can imagine instead of getting to trials, 1/2yearlies or even the HSC and going "shit i should've actually listened in class" trust me it helps ALOT
4- depending how you study {im a writer} i wrote essays over and over and over. Although for my story in Adv. English i wrote one just from the top of my head, although it was 8 pages. So just make sure you know your stuff.
I recommend, one on one study sessions with your teacher/mentor, if your school has that set up if not, talk to your year advisor or deputy (depending on the size of your year/school) and see if that can be set up. I had my Bio and English teacher as mine, and it helps ALOT, we had some group sessions set up for Bio too cause it was such a LARGEEEEEEEE topic! and it is always good to have someone to talk to who's abit older (although my English teacher was 2nd year out of uni, which is awesome but yeah).
When it comes down to it, study for a few months befor the HSC (as ridiculous as it seems) i didn't haha. it wasnt the best idea, but i did do well,i was doing 8hr days sometimes though and crammed ALOT ALOT ALOT! haha. and did like 7 essays to different questions the night before exams as well.
If you can (which it is hard) avoid drama, although some you can't help, but with relationships etc, it can suck when there's a little fight the night before the Paper 2 Adv. Eng. HSC exam *speaks from experience*, but DON'T become a recluse and think that the HSC is the only way you'll ever to what you're meant to. It's BULLSHIT! (excuse the language) when people put soooo much pressure on you guys to get 100% or do subjects "just cause they rate better with UAI's" cause its NOT TRUE! and being a recluse and studying every day (although a small amount is good) it will burn you out, and you will feel like there's no point, which isn't true, but just take it easy, go out on the weekend and such, just dont get blind drunk and do drugs every weekend or at all, it wont help you both physically, emotionally and mentally.
AND there are back ways into uni and tafe and you can always repeat (although put in the effort, repeating with people who aren't your friends WILL be alot harder).
Hope that helps a little bit; Good Luck and you can ask me other questions if you wish or PM me, i'd be happy to help.
and have fun!
OH! and past papers, yeah they help ALOT get you used to the broadness in the questions etc. but my HSC (2008) had VERY different questions in my subjects (Eng Adv, PDHPE, Food Tech, Chem and Bio and Gen. Math- not so much math) so any trend which you saw was non-existant which did help.
E.g, for foodtech, every year they had a large question on Legislation and Laws (you foodies will know what i mean soon) but it was in only ONE multiple choice question, although my whole class at the end was cheering it can be like that. So just remember just cause they asked it in 2006 that it wont be exactly the same, but yes, do do them when you've done all your study notes etc, and need to start practicing questions.
(ALSO! haha, this is getting too long sorry)
USE THE SYLLABUS! it helps more than you can imagine, although not so much in english, but writing a page or so on each little point helps ALOT! and you'll have comprehensive notes in the end, this you can do from Day 1! just do it for homework, put it away in sections and tick it off, when you've done one whole course eg. in Biology, the relief is HUGE! and it makes it sooooooo sooooooooo much easier to study and to have it all there, i ended up writing the biology course out twice and chem out twice too.