First of all before sitting an exam practice not only writing essays, but also writing plans. A good plan should be detailed and you can take 10 minutes at the start of your exam to make a thorough plan so that your essay is easy to read and structured, giving a great first impression to your examiner.
Two Key Methods
There are two key ways to look at an essay, generally speaking, in an English Literature exam. You can either look at your text, or texts if you're writing comparatively, thematically or chronologically. Personally I prefer the former way as this avoids writing in an overly descriptive way. This is also how essays are written at a university level and show the examiner that you know the text well and can analyse or critique it as a whole.
A Thematic Plan
When writing your plan keep referring back to the question, what am I being asked? Look at which themes or ideas relate to the question. For example if you are asked 'To what extent is 'Brave New World' a purely dystopic view of a future world?' you may want to use key events or ideas to analyse this. Start with key ideas, in this case indoctrination, then focus in on specific characters or events, in this case Bernard's state of depression through realisation, and how this answers the question. By doing this for each paragraph and theme in your plan you ensure detail and focus on the question.
A Chronological Plan
This kind of plan can be useful when looking at a piece of poetry or a short unseen text. However be careful to avoid being overly descriptive. This kind of plan is easily structured, something slightly harder to do with a thematic approach, and can often bring you to a clear conclusion. With a piece of poetry start by looking at the overall structure of the poem and then focus in verse by verse. Each point should have an example, analysis and then a deeper look at why this is relevant to the overall text. Why does the rhyming scheme show a sense of melancholy? Why is the repetition of the word love significant in the same verse?
Final Top Tips
When writing a plan always ask yourself more leading questions to give good depth to your finished essay
Don't panic, analyse and breakdown key sections of the question
Look at the point of the essay and whether what you are looking at will be relevant in making that point