Ah how interesting! You must be one of Christina's mates.
In a nutshell - keep the HSC in mind, but don't obsess. I would say to take it easy and chill out until the end of Year 11. Do subjects you really enjoy - I found the HSC extremely easy because I actually LOVED every single subject I did, so studying didn't even feel like studying - it was just plain fun.
Study tips, well it depends. I was able to write Excellent+ essays the day before, some people need a week of polishing and drafting. I also had an affinity for the conceptual analysis of History and history extension - so that worked for me and I didn't have to really "work" for it. It really does depend on the subject and the person.
I have to emphasise that what pymble teaches you is to really be a well rounded person- the fact that it was never just about my marks, it was about how much fun and how much I could achieve outside of academics in all my extracurriculars. Keep that in mind.