Writing a great answer to an AS or A2 English essay is tougher than it was at GCSE but it is important to build on the skills you developed at the earlier level.
The three things to remember are
a) Focus your answer - make sure you are answering the question!
b) Structure your essay clearly
c) Impress them with higher level requirements e.g. language analysis, context and maybe even some critical views
a) The most important thing when answering a question in an English Literature exam is not going off topic, trying to include all the great material you have revised even if it doesn't fit! Take at least 10 minutes at the beginning to write a plan, a few bullet points will do but make sure that these are all fully relevant to the question.
b) In terms of structure, even if the essay title has the term "discuss" in it, it is good to come up with an argument. Try bullet pointing 3 or 4 ideas on one side of the argument and then coming up with 3 or 4 counter points. This forms a cohesive structure for you! Then all you have to do is outline both sides of the argument in your introduction and try to come down on one side of the argument in the conclusion. Remember, the conclusion is the only time in the essay that you can use the word "I" and give a personal response, so try to make the most of it.
c) To impress the examiners and show you are now a better English student than you were at GCSE you need to develop your analysis skills. At GCSE you may have been taught the P.E.E method (Point, Evidence, Evaluation). At A-Level try to build on this by making your point, giving evidence from the text (a quotation will always impress), evaluating what the means for your argument AND including some close analysis or a historical or social contextual point in that evaluation.
For example, in an essay on Edmund in "King Lear" I might write: "Edmund has clearly suffered injustices throughout his life due to the position illegitimate children were in. (A point is made and extended via contextual knowledge) His bitter attitude is evident from the harsh language he uses, for example, "why brand they us with base? with baseness? Bastardy? Base, base?" The repitition of "base" and "bastardy" with harsh, plosive alliteration shows his fury at being considered inferior due to the nature of his birth. (A quotation is selected to back up the point and then the language is analysed to draw out further implications)
Think about these three basic points next time you sit down to write an essay and see your mark rise!