Work out where you are going wrong in the exams. Go over topics you are unsure of if you keep losing the marks in the same area. Also remember to check your answers to make sure you haven't made any silly mistakes.
I think you just need to keep on doing past papers. I'm not sure if that'll guarantee you an A*, but the questions in past papers tend to be really similar and are always about the same topics. If you understand the concepts then you need to do past papers to learn how to apply the concepts in a variety of ways.
I know this information is a little useless but it's really the best help I can give! I hope it's what will give me an A* this year too.
The way some questions are worded throw me off sometimes. Also, with some questions I don't know how to actually approach them. Question about shapes and algebraic proof.
I also do drop a few marks from silly mistakes, so I need to get that out of my system.