Read. Read constantly, obsessively, and intensely. In the car, on the bus, in the bath, in boring classes. Don't whine about it, it's good for you. And the great part is, you can read anything you want! Loathe the nasty, wordy classics? Make it your goal to read one every semester, but then read young adult fiction, or mysteries, or even news magazines like Time or US News and World Report. When you read, you soak up the words, the syntax, the way that the author conveys the tone. With a good author, you don't have to be consciously paying attention to absorb their skill.
2
Study your grammar. No, you don't have to spend hours diagramming sentences. If you have accomplished step one, you should have already learned by osmosis why some sentences sound or look right, and others don't.
Grammar, writing structure, and vocabulary all make the difference to scoring above a 6.
3
Pick your topic. Now that you've dressed yourself in great vocabulary and an understanding of grammar and syntax, it's time to pick a decent restaurant; your topic, or in the case of rhetorical analysis or persuasive, your stance.
Write with passion. If it's an open essay, make sure you write about something you are passionate about. If you're bored, it will show in your writing.
Ask for help. Ask your instructor whether your thesis is good enough. Your entire essay will be built around this, everything must relate to it.
Make sure you are answering the prompt. If you do not answer the prompt if given one, you will get at most a 4. Misreading the prompt is a 5, and writing at a standard writing level answering the prompt is a six.
4
Build your paragraphs. Now we're to the "meal"--the most fun, but also the trickiest part. You're going to cut it up and eat it; that is you're going to let the inspiration flow and build up paragraphs around your thesis.
Use excessive evidence. In rhetorical analysis, if you're going for an 8 or even a 7, there is no such thing as too much evidence. If you are truly striving for a 9, you might get back feedback like "overkill".
5
Create a nice tight conclusion. Your chance, as the chef, to show off.
Be creative. Teachers have already read your essay, so you don't have to summarize it again. Revisit some points, but don't retell it.
Don't reinvent. Definitely don't bring up anything new. If you've found yourself doing this, probably make it a whole different paragraph.
Make a general point. If you can find some way to be novel, wise, or basically make yourself sound worldly, do so. Somehow relate all that you've written to the universe.