Planning is key here but it is important to remember to balance your time between planning and writing your answer as the exam will be strictly timed.
Think about what the question is asking - it is important that your answer has a strong argument and to do this you need to work out the type of question. It might ask whether you agree, in which case you need to agree or disagree, or it might ask you to describe, then you will have to evaluate the facts.
Avoid chronology - if you try and write an essay in the order of the events, you will end up just writing dow what happened. To avoid this plan your paragraphs into arguments. If you begin each paragraph with a strong argument you will avoid this. Remember examiners know what happened, they want to know the reasons and that you can think for yourself.
Stong intro and conclusion - these are essential and why it's so important to plan, as they should mirror each other, they should agree. The conclusion should also be decisive, never sit on the fence!! If you start and end your essay well, it will impress the examiner from the beginning, also it will put you in the right frame for your essay, so will keep you on track.
Refer to the question - throughout your answer keep looking back at the question, and even better keep using the language in the question - this will keep it in your mind so will keep you from getting distracted - you only have a short time to plan so it's important to keep going back to the question in case your plan is only really brief.