Honestly, in addition to doing tons of past papers as quite a lot of posters here have told you already, I guess the only thing that would make sense would be trying to find a way to double-check or even triple check your answers. This could be just doing the question again, or thinking outside the box.
Also, from experience, getting that A* comes from your ability to have some sort of knowledge in maths generally. For example, there are certain rules you should know and why, and how certain topics can test these rules. Theres a lot of knowledge crossing in maths, so ensure you actually understand the topics and don’t rely on route memorisation.
To get better understanding on topics I highly recommend TLMaths. Check out his YouTube, it will take you to advance level. I got A* from this. Hope this helped in some way.