How do you structure a top-marks essay?

When structuring a top-marks essay at GCSE (and beyond), it's important to remember that you're trying to explain and discuss a topic. You're trying to show the examiner (or your teacher) that you have a wide range of knowledge about the topic you are answering. This is important to keep in your mind when responding to a question.
So, in terms of structure, you want to be clear and concise. Begin with an introduction that incorporates the words used in the question so that it's clear to the examiner that you're answering the question. Explain what you'll be talking about (usually two points for an argument, and two points against). Begin with your weakest point, and gradually build your points, leaving the strongest until last. Throughout the whole essay, keep coming back to the question and give each paragraph its own mini-conclusion.