I'm currently undergoing my A-Levels, but my GCSE English Literature & Language grades were both A* and I credit such grades mainly to leisure reading as well as browsing forums and engaging with others.
Knowing what the examiners are looking for and being able to understand the exam question is key to achieving a high grade. Remember, in the exams - there is little to no help available (the most they can do is read out the question to you) so if you don't understand how to answer an exam question, then you may spend all your time being misguided and achieve a low grade as you done something other than the specified. The way to go about learning how to answer exam questions is by doing practice papers and learning about different language techniques and using as many as you can.
SPAG (spelling and grammar) also play a role in making sure you achieve the top grade - so make sure you use correct grammar, punctuation and sophisticate your language. For example, which sounds better?
A) 'I had earned his cordial loathing'.
B) 'he liked me because i was nice and friendly'.
And read assigned material (books, for example) two-times over. I always find things that I didn't initially notice upon second reading.