what are some good tips for writing a 20/30 mark essay?

When writing a long essay, it can be easy to waffle and go off topic. It's important all your points are relevant to the question. You can make sure of this by always linking your point back to the question at the end of a paragraph. Furthermore, you want to make your argument flow well. This can be done by always linking your new point to the previous one. A conclusion is always necessary to sum up your ideas and give an overall judgement- try to give a firm judgment rather than sit on the fence- again use language from the question to ensure you're properly linking back to the question.