The biggest mistake people make when preparing for LAMDA Examinations is not properly reading the syllabus. This, along with other materials published by LAMDA, is your main resource and contains everything you need to do in order to pass. There is no mystery or hidden trick to gaining that Pass, Merit or Distinction; everything is laid out clearly in the syllabus (made particularly clear by the recent change in design).
Because of the nature of performance exams (practical, one off exams, with a degree of subjectivity) it can be easy to think that the syllabus isn't very important. This would be a big mistake. When I work with learners I always have the relevant syllabus to hand and am constantly referring to it. After all this is what the examiner will be using to assess the learner.
Remember the syllabus is designed as a practical tool for the teacher, so use it! And if in any doubt, contact the LAMDA Syllabus Team. Although they can't offer teaching advice they can help clear up any questions you may have and are super nice and supportive!