Look at the bits you did wrong last time. Do them right next time. Look at the bits you did right last time. Do the same thing next time. For the practical bit you just need to be efficient and accurate, for the questions you just need to learn what ways your exam board is screwed up in, and compensate for it. If you make an observation that's different to one you have been taught go with the one you've been taught, not the actual observation (it's dishonest, but it gets you the marks).