For scoring a high score (280+), first of all you must have clear concepts. All topics must be studied well. No concept is to be left untouched. Questions can come from anywhere and so you must be prepared for everything. This requires immense revision during the two year preparation. All reactions, formulae, question type, etc. must be revised until they are at your tips.

Also you must be good at solving problems in examinations. For that, you need to practice a lot of tests. After your syllabus is over, do full syllabus tests every few weeks. This will revise all concepts and make you confident about speed and temperament.

Most important of all, don't mess up on final day. Solve easy questions at first, then ...