There are as many answers to this question as there are writers of essays and arguments to be argued. Say what you mean and mean what you say. This might sound simple, but it's the hardest thing to make your writing do. Present your argument in a clear, coherent order. Don't jump from point to point without telling your audience where you're going.
Writing is a process.
It involves thinking, reading, writing, rereading, rethinking, and rewriting. It's a grueling, painful thing, especially when we have to throw away something that we've already written, but it's the only way we can really know what we think and communicate that thinking to others. First and final drafts must be typewritten or word-processed. Use only 10 or 12 pt. Do not justify on the right hand side.