I used a book about writing essays to get into medical school or just the section about essay writing from the guide to medical and dental schools. Sorry I don't have the books with me in my present location.
Whatever you write, just don't write stuff like you went to a dentist and/or orthodontist for dental work, you appreciated their work and you fell in love with their work.
when I was in the interview at NYU, the admission lady praised my essay for being original and not like the idea mentioned above. So be creative. Write something about yourself and it needs to show that you are a motivated person and deeply deeply in love with the dental profession .
Also, try professional editing service on the web. I found they're very helpful
Reading application essays in books and on the web made things harder for me because I found myself wanting to write what was already written by someone other than myself. Just try to write a story that indirectly shows your strong personal attributes like motivation, integrity, compassion, etc. The essay doesn't have to be about dentistry, just try to relate it to dentistry in some way. Once you personalize your essay, professional editors can help make sure the essay is written well.
If your undergraduate school has a writing lab where people help you write essays, papers, etc., I would recommend going there for some help. I know at my undergrad school you could go for anything (not having to relate to class) also, you could even just go for a brianstorming session with someone. This is something that I used to help me write my essay.