I received final marks of over 90
my HSC courses so I believe I am qualified to contribute to this thread
To keep it short and simple the greatest tip I have for those aiming for a 99+ atar is SELF-DISCIPLINE
Studying on your own or without the guidance of others can be really difficult. I learnt to hold myself accountable during my study sessions and to study as much as I thought I would need. I found out what study methods best suited my personal needs in Year 11, which I took as an experiment year. (Don't take Year 11 for granted! Work as if you're preparing for the HSC that year!)
Personally, I spent about 4-5 hours a day after school doing my notes, revising and completing past papers when it came for Trials and the HSC, but this would vary so I would spend more hours on the weekend and more hours (9-10) in the holidays or just before my HSC. Being consistent is one of the best tips I can give in terms of being able to retain information in the long-term
so that you wouldn't waste days of trying to re-learn concepts you've already been exposed to earlier in the year. Whether that be making flashcards, condensing your syllabus notes so that each topic is max a page long, or even reading aloud to yourself your notes every, single night before bed. I would say I was pretty religious in reading my notes and I would even bring them, ngl, EVERYWHERE I go. To the shops, on a cruise, overseas, wherever. I remember the day before my English Paper 2 exam I was at Woolworths shopping with my mum and I was pushing the trolley while holding a folder of my essays for each module in my hand. People stared at me obviously but just shows how much I was commited to study and to earning that 99+ atar! But take these tips with a grain of salt. They might not work for everybody!
Oh, and one more massive tip: Make your own notes!
As tempting as it is to use notes from a former Band 6 student posted on forums such as BOS, they will never amount to how valuable making your own notes and summaries will be. Trust me. Don't rely on others to bring yourself up. Work hard! No pain no gain!