I don't know if your exams are similar to the exams we have in the Netherlands. What I did was making the exams from previous years. Every year they always ask the same types of questions e.g. every chemistry exam has at least one question about redox and pH. By making exams you learn about the way the questions are asked and what information they want from you. Basicly I never did homework or paid much attention for years, and by making some exams a few weeks before my final exams took place, I scored about 80 average for my exams (Dutch, English, physics, chemistry, maths and biology).