As said above, studying in accordance to the syllabus is crucial. However, I also found that reading widely helped me dramatically, since I did partake mostly in humanity subjects.
Academic excellence is not only studying the textbooks and the syllabus but widening the parameters of your research. This applies especially with English. I read philosophy during year 11 and 12 out of my spare time or even when I was studying, not only was it stimulating for the brain, but I soon came to realise how invaluable it was to my analysis skills. The way you think and perceive is enhanced greatly, no doubt, and when in exam conditions you really shine. All that I had read which had no correlation to the syllabus whatsoever, came together during exams or during assignments, weaving threads and making connections. In whole, it allowed me to have a different take on certain subject matters which differentiated me from the rest of the students. Also, what you write actually carries A LOT of substance and that's where most of your marks come from.