It was only at university level that I learned to write a high quality essay. I now abide by the following:
- Structure: most students know that an essay should have a beginning, middle, and end. But it's easy to get carried away in your writing, and forget that the end should respond to the beginning. Your introduction should offer your essay plan, and your conclusion should pull together your various points. A good essay is structured as follows: what you're going to say, what you're saying, what you've said.
- Concise writing: if a sentence can be made shorter and retain its meaning, do it. I know I was prone to fanciful phrasing, and without it my writing is clearer and sharper.
- Evidence-based statements: avoid sweeping generalisations. Be very specific about what you want to say, and support it with evidence from the text.
- Confidence: don't hedge your bets. It's fine to say a text is strongly x or y, or equally that it's a combination of x, y and z. But you don't want to water down your writing with assertions that a text might perhaps be this or may be that. Be confident in your answers! As long as they are well-supported, they are perfectly valid.