If you're just starting AS then I would say don't just aim for an A, try and get full marks in every paper. You might not achieve it, but it will help you out a lot.
If you're starting A2 then it depends on your situation. As you need 90% average in C3 and C4, by achieving them I only needed 20% in my third paper to get an A*. Work out if you are in a similar situation as then you know you can focus less on that one, and more on C3/C4
Practice and keep practicing. Don't limit yourself to past papers from your exam board, do other things such as IYGB Papers because even though the questions might be structured differently to the ones that will come up for you, it tests that you are able to apply your understanding of the maths
Good luck!