It may sound cliched - however to get the highest marks in Maths at A levels means going beyond your exam board! Do not just rely on the past paper questions from your exam board but try a few papers from the other exam boards! Especially since now more than ever students are being asked to apply their knowledge rather than follow a series of calculations. Hence fully understand the material - what and how the solution is approached and when it can be applied. Also a strong recommendation to getting 90%+ is by trying the notoriously tricky Delphis and Solomon papers.