Making sure you answer the questions being asked , that your language and mechanics are good and you know to tweak things to suit the question. That is what i have been advised to do to get a top grade.
To become good at writing I'd suggest reading anything and everything throughout the year. Pick up books in a second hand shop for a euro, read them cover to cover, and you'll find your vocabulary expands and your sentence structure improves. You can know all the quotes you want put if you can't write freely and naturally it won't matter.
In terms of questions, definitely make sure you stay on track and don't digress from the question being asked. Constantly refer back to the key words in the question. Avoid summarising your poems/texts, stay on topic.
And IMO on Paper 1 composition avoid the short story like the plague and opt for speech/personal essay
It is so difficult to get an A. Aim for a B 1 if youre counting for points and anything extra is a bonus. I went on to study English in college, worked as a writer amd am now teaching English, and I only got an A2