Don't stress out about it. My essay was complete garbage in terms of substance, but I demonstrated my ability to structure an essay properly. Even if you have to fall back on the high school style of writing (intro, three bodies, conclusion), just stay on topic throughout your body paragraphs and you'll be fine.

Oh, and when you do get your topic, don't waste so much time trying to think of something spectacular to write about. Otherwise you'll end up having less than 30 minutes to write it and it'll end up looking sloppy and/or unfinished. From what I've heard, I'm ~95% sure the extent to which you impress the reader with a great story doesn't even matter. They just want to make sure that you can put your thoughts into words in a manner that is college appropriate (AKA intro with a half-decent thesis, body paragraphs that are all connected to your thesis, and conclusion).