My teacher said the best way is to plan it like a proper essay, but you've got to make sure that everything you say is important (ie. not just filling space) cos you don't have much time to write it. Your school should have standards packages for every subject, they're like examples from the different bands so you can sorta see what kind of response would get the highest marks... and yeah you need to know the pieces heaps well, and also have "quotes" from them, like in english - memorise a bar or two as an example of a specific technique and practise writing it for the exam... yeah that's all I can think of.