I got the grades you are hoping for (A*A*A*A) and I found that making very thorough notes on the revision guide helped me the most (basically copying the entire thing out 3 times). I also used the specification as a check list to see if I knew everything, sometimes exam boards like to hide things in the specification that they leave out of the revision guide. Lastly around 1 month before my exams started I did every past paper 2/3 times. This was probably overkill but what helped the most was using the mark scheme to mark it myself. This way you can see all the possible answers you could have given to the question.