Read the prompt carefully. Know exactly what the prompt is asking for and what details you must include in your essay. It does not matter if you write the most perfect essay in the world, you will fail this portion of the test if you do not answer with the information the prompt is asking for.
Brainstorm ideas. Write down whatever comes to mind having to do with your prompt. These ideas do not have to be in sentences, or even make sense to anyone but you. Do not worry about spelling or punctuation, just get the ideas down.
Freewrite your thoughts. You may find that just writing your thoughts down as they come sets you up for a great essay. Just let your mind whirl and try to keep up. Once again, do not worry about spelling, punctuation, or run on sentences.
Create a cluster diagram. Write the main idea for the prompt in a circle in the middle of your paper and start branching off into your other main topic sentences and smaller details. You may find this visually easier to collect your thoughts. It is also a great way to see how much support you actually have for each topic.
Make an outline. If you already know how to do an outline, this may be the best prewrite for you. Set up and organize your paper as you think is appropriate using this strategy.