My method
The answer is genuinely simple. But you have to get the most out of every single question. Here are the four steps I followed for every topic.
Get the fundamentals of the topic down properly. Don't talk yourself into thinking you understand when you don't. Everything else is built on this.
Do a couple of questions from your maths textbook to get comfortable with the topic. Don't spend too long here. It's a warm-up, not the main event.
This is where you should spend most of your time on each topic. Do as many as you can while making sure you understand every one. If you don't understand something, get help. That's where tutors and teachers come in.
Sit timed exams under exam conditions and get them marked by a teacher or tutor. Then work out exactly what you still need to do to perfect your maths.
Give it 100%. Sit on the same question for 30 minutes if you have to, trying different methods.
This happens a lot in Extension 2.
One topic, start to finish
Steps 1 and 2 happen before your teacher gets there.
Because you've seen it already, class strengthens your understanding and you can ask better questions.
As many as you can, understanding every one, with help between lessons when you get stuck.
An exam under real conditions, marked, so you know exactly what's left to fix.
The lesson then reinforces what you already know, and you can ask better questions.
Study with others who also want to do well, and work through questions together.
Know the mark you want and what you need to do to get it.
This is how I got a 94 in Extension 2 and 96 in Extension 1, and I used it for nearly every subject to get a Band 6 across the board.