I think writing practice essays and getting them marked is really important. You can do this by handing them in to your teachers - but some teachers at some schools tend to get annoyed if you hand too many in. Try getting an english tutor? Or at least an essay marker.
To maximise your marks, you really need to memorise a strong essay - it doesn't have to be word for word. In fact, it shouldn't be. But you have to know what you're going to say, what points you want to make and what techniques and quotes you want to use. In the exam, you need to ANSWER THE QUESTION. Don't just spit out what you've memorised, but make sure you use what you've memorised to answer the question. That's the single most important thing to do. It sounds so simple, but so many people get it wrong. You can't just waffle on and on about some thing only tangentially related to your argument.
Good luck!