Unfortunately there isn't some big secret that people who get 99's all share. You need to study the material and work the practice MCQs.
Also, basic test taking strategy will help you out a good amount. 60% of the exam is multiple choice, which means the correct answer is given to you. You don't always need to know if off the top of your head. You do however need to know enough to weed out the obviously incorrect answers and get down to the 1 or 2 remaining choices. Avoid answers that have absolutes (Always and Never) when possible, unless you are positive that is the correct choice.
If you can begin to think like the test creators, you'll have a better chance at understanding why they are putting certain statements into each MCQ. Doing that will make it much easier to eliminate any remaining distractors. Keep an eye out for buzzwords (if a question states or implies anything to do with managing, or management responsibilities, more often than not an Auditor is NOT allowed to perform those duties).
Also (and this part is very important), there is a bit of luck involved in scoring a 99. I don't think anyone has truly mastered all of the material tested on any section of the exam. If you're lucky, the stuff you don't know or don't know well will not show up.
Make sure you are familiar with the authoritative literature/standards. I don't mean memorizing different sections, I mean being able to navigate to material and use the search function effectively. I had at least 2 SIMs that the full answer was provided in the authoritative literature. Using the search function I was able to basically copy paste the correct answer into the SIM response box. This had nothing to do with my understanding the material and everything to do with my being able to search effectively.
Hope that helps
AUD 99
BEC 96
FAR 94
REG 96
.