Prepare yourself globally for exams - try to learn to work out what EXACTLY the examiner is looking for - every Q is designed to test certain specific topic{s) in the syllabus - you need to work out which topic it is, then you will be able to work out what is in the mark scheme (without looking at it):
1. Read the Q carefully twice
2. Try to remotely read the examiner's mind.
3. Look at the no of marks allocated to the Q - put down at least as many facts/ideas/points/reasons in brief.
4. When you write the answer imagine you are saying to the examiner "Sir/Miss, this is what you were looking for - that is EXACTLY what I have put down - can I have 2 marks (e.g.) please - thank you!" Go to next Q.
Look at a few mark schemes and try to work out by looking at the Qs how your train of thought would have led from the Q to the A - don't just blindly keep doing past papers if you are not getting anywhere - find the connections between Q and A.
Look at my attached article - gives some tips on how to write high-scoring answers.
Best of luck - if stuck on 1 or 2 specific Qs, pls post here and I will be happy to help out.
M (Specialist Biology Tutor)