How would I prepare for my Maths exams so that I get the best grade possible?

I would recommend that all A-level students should attempt all previous exam papers at least once. This is to pick up on common tricks that the examiner uses and in most cases these tricks keep coming up each year, such as x^2 - 4 being able to be factorised to (x-2)(x+2). Another useful thing about doing past papers is that questions answered incorrectly can be reattempted until not only the right answer is achieved, but the student understands what they got wrong.