Grab one of those practice books that have 12 SAT tests. The 4-6 weeks prior, take every test (ideally in one sitting), and don't skimp the essay prompts. Allowed me to get an 800 on Math, 690 CR (from 600). Writing had stayed at 620 because I didn't do the prompts. 
Are schools actually looking at the essays as something important yet?
Honestly, I think they are generally used for tie breakers or covering up a soft blemish in grades. It's a shame, because the essays are generally a more indicative measure of intelligence and character than GPA. It's a test to see if a student can articulate themselves and what they can offer to the school. Shame that GPA still has the highest weight in admissions: cheating was (who am I kidding, still is) rampant at my HS, and I imagine that's pretty universal.
Although I like to think that my essays snagged an acceptance at William & Mary when I got rejected near everybody else 
, or maybe it was the 2110/33 scores. One bad semester can deny you the Top 20s, but it is what it is.
I think I've wrote some good essays(At least according to my Ap Lang teacher and college comp prof), but they can easily not be genuine, and people cheat on those as much as GPA. I don't think you would recognize my first and final essay drafts as being written by the same person thanks to the editing from my the teacher I mentioned. And of course there are essay writing services where there is no input at all from the student.
