I took humanities/social sciences so this may be more relevant for those students:
-Only take subjects you are genuinely interested in, not subjects you happened to do well at during GCSE
-Be prepared to mature significantly in your writing and learning styles, A level is a big jump up from GCSE. I have seen the jump catch people off guard pretty badly
-Complete all set work on time or within a reasonable time period, feedback is absolutely essential for A level exam success
-Listen to and take seriously the feedback given in mock exams, it really does matter at this level
-Embark on study of your subject that isn't included on the mark schemes, especially for humanities subjects. Examiners love nuance points and will reward them. For example, I included quotes from a book on globalisation from a theorist that wasn't on the mark scheme, and got extra marks (Politics)
-Start revising during the Easter holidays, find a revision technique that works for you. Past papers are essential regardless of the subject, and go over the mark schemes as well. In the month leading up to exams, you've really got to be putting at least an hour in every day, and then 4 or 5 at weekends. This is important if you are aiming for the high grades.