Instead of telling you what constitutes a “6” or a “5”, or even a “0”, I am going to provide a nifty link to a document the College Board has prepared. If you go to the 6th page of the doc, you’ll find a nice description for each score.
This resource just provides a description, and doesn’t really give a sense of what counts as an actual “6” essay, or an actual “5” essay. The best source for actual practice essays is the College Board book. Here you will find actual student essays—essays written the day of the SAT, semi-illegible cursive and all. The link above also provides some examples.
You’ll notice that there is a range of styles, and not every one of the stronger essays has a cookie-cutter organization structure—though most do. You’ll also note some personal examples in the higher scoring essays, showing clearly that the graders are not biased against personal examples.
Of course it is hard to grade your own essay, even if you compare it with these others. If you are taking an SAT prep class, you have someone who can give you a score, and, hopefully, helpful feedback. If not, ask friends or teachers for helpful feedback, even if they are not that familiar with SAT scoring scale.
Finally, remember that the essay takes a lot of time. While it is a noble thing to want improve your writing, so that your “8” becomes a “10”, all the hours it will take to do so, will only result in a 40-point increase. Unless, you are scoring near perfect in all the other sections, you should spend time brushing up on your math skills or your grammar skills can quickly net you 40 points. I should also note that if you get every question *write* (Chris Lele pun…) in the Writing section—no easy feat, of course—an essay score of “9” will still earn you a perfect 800, or at least a 790, depending on how the test is scaled.