there's a whole free essay workshop thing right here on SDN (look at the top toolbar, under Applicants). It is excellent.
I would have been better off doing what blee suggested; I write well enough that I did not need feedback on grammar, spelling, style, etc..
Most of the books I looked at were filled with sucky essays that were trite and uninteresting. Going through the process of workshops with the state school premed advisor, having ppl read it, etc. just resulted in me having a trite, boring essay that sounded like everyone else's. Most young premeds don't have the skill or expertise to give good feedback in a peer-review workshop. The people who run these workshops are used to regular pre-meds too.
I had an expensive premed advisor read it too, and that was so discouraging that I wished I hadn't. She didn't give me any good specific advice, just made it sound like I wouldn't get in anywhere.
In retrospect, I would have been less afraid about trying to fit someone's vision of a good med school applicant. I would have left out much of the justification of why I'm going to be a good med student, and I would have focused much more on my interests and passions. I would have gone more heavy on the first-generation college student, no doctors in the family. And so on. I did get interviews almost everywhere, but I missed UCSF, which I later found out only looks at your essay and the titles of your experiences (not the descriptions).