It's always doing those past papers over and over they would say, but how you do it is a different story.
If you have trouble remembering stuff - don't time yourself. You are going to panic, feel over-whelmed and skip the question just because you don't remember the formula (if you are doing closed book). If you are doing open book, it'd honestly be just a waste of time going through the textbook. My tutor told me to get a small notebook and call it my formula book. Once I understood the content in class, I'll write all the formulas and key points in the book and divide each section by topics (I can upload pictures if you want). Now I don't carry my textbook and I only use the formula book for reference on minor geometry formulas (because we all hate Question 16).
Another method my teacher told me (I tried using this, but I never do it), is grabbing a piece of paper, clearly write the formula an key points on the paper and then sticking it somewhere you most commonly look. This might be (as I quote from my teacher) "the back of the toilet door", a pin board, a mirror, the fridge, just anywhere is fine. Constantly looking over and over the same thing will get integrated into your mind (ah, math puns).
Now I often have a photographic memory, but that isn't to say I don't remember all the formulas. It took me quite a bit, but not long. It just involved a lot of going over the same questions over and over. When you answer the question, write the formula first (MUST ALWAYS DO THIS). I've noticed in my class that when people don't do this they lose marks. However there are always those minor things that comes after the formula such as the +C when integrating. It'd be wise to put the +C when writing the indefinite formula but I never do (and that has cost me to be forgetful, but I never lost a mark from that mistake). Also there are little things that I make up along the way to help me remember, for example, when integrating sin, I remember it as SIN - Sin Integrates Negative. To tell the difference between Simpsons and Trapz rule (cbb spelling), I just remember that the Simpsons had 3 kids, therefore at the beginning it is h/3. If you can find connections between math and something else, I trust you'll be able to remember things much better.
I don't really have much to say about the working out process except for DON'T SKIP STEPS. Please don't. So many students lose marks and get things wrong because they forgot steps because when doing practice they wouldn't include the steps and thus when it comes to the exam they have no idea what's going on. Just take it easy. Once you've established a foundation, start speeding it up. Improving speed and accuracy is vital for HSC Maths.
Anyways that's all I have to say so hopefully these are helpful
Good luck with Year 11!