GCSE:
Maths: A
Additional maths: A
My individual modules at AS (UMS)
C1: 98/100, A
C2: 78/100 B
M1: 92/100 A
At A2 currently working around A's, few A*'s.
C1 is the **** easy exam normally (sorry if I make it sound like I'm bragging) and is the non calculator paper. C2 and M1 I would say are harder (and I hated last years paper for C2 clearly). During the exam season I did EVERY SINGLE past paper my college had available and in one of the exams it went back to 2005.
Early on in AS I was quite lazy as I basically did the homework and basic textbook revision but I got a range in homeworks and class tests of D's to B's. When I started doing past papers as revision (not loads at first) I was getting to A's. Basically do lots of past papers because some of the questions are hard in comparison to class tests.
In A2 apart from C3 and C4 I'm doing D1. This year is pretty much the same in revision except that exam questions are far more horrific if you do not do past paper revision. There was a long question which then said the following: show that cos(4*veta) + 4cos(2*veta) = cos^4(veta)+3. That question first time was horrific for me and I got it wrong with like 15 lines of workings. Did it several months later with exam practice and I did it in about 5 lines.
Maths you need to understand the formulas and how something works and then being able to apply it. If you don't understand something that you learn you need to clear it up with a teacher in a workshop etc and then you need to understand how to apply it since exam questions will make it far more complicated. If you struggle to understand topics and stuff already in maths you will have a very hard time at A level.
Advice for revision: if you have one of those large textbooks with knowledge and recap questions along with exam style, skip the knowledge and recap questions. I also wouldn't even use the textbook that much to read up on knowledge, just ask your teacher if you don't understand something. In A2 (can't say much about my AS XD) my very early revision used the textbook, later on (and including later AS) I NEVER used the textbook for revision.