How best to maximise marks in exams, for example in definitions or in 20 mark questions

For any question the you should first consult the mark schemes and ensure that you are understanding exactly what they are saying. For a definition question repition is key, making sure that you understand the key parts of the definition are mentioned and explained. For a longer question, one must ensure that the diagrams are all correct, that each part is carefully explained and that all of your explanations are clear. You must develop your answers but in order to do so you must ensure that you have a grasp of all topics. The best way to do this is to practice as many questions as possible, each time learning timing and new technique.