How should I plan my essay?

First, take a deep breath and read the question three or four times. It's easy to get flustered if the topic and/or wording is not something you were expecting - it may be an area you're not comfortable with at all. The worst thing you can do is panic and start writing immediately; doing this is a sure-fire way to writing an incoherent answer. Focus specifically on the questions wording and identify two or three key words. It's important to identify the subject matter (usually the noun, e.g. Bismarck) and then what they want you to show, which could be the extent to which something is true or an explanation of how something happened. Deal with the noun first and note a few key events or historical interpretations. Try then to organise the information in a logical manner, ideally one topic per paragraph, otherwise you'll be covering too much ground.
This preparation will help to give you clarity when you get to the analytical side of your answer - the extent to which/why - which is where you can really boost your grade. Once you've come to a decision about how much you agree/disagree with the question, write the word 'argument' in capital letters at the bottom of your plan, and then write your point clearly. This is good for both you and the examiner. At the end of each paragraph turn back to your argument to ensure that you are tailoring your evidence to support it. This will ensure that your essay is coherent. What's more, if you run out of time - which always happened to me! - then the examiner has a clear idea of sophistication of your argument, and what you would have gone on to say if you had a few extra seconds. The likelihood is that this, accompanied with the fact that the examiner will be more likely to understand the essay throughout, will boost your overall mark. This advice applies to other essay-based subjects, such as English.