copy&paste from someone else's post
This was how I did mine:
I sent mine out near the time when they were deciding my application (they sent out acceptances every 15th of the month):
1. First paragraph, I introduced myself, gave a brief background. I'm nontrad and had an unusual background, so I figured by stressing my background, I would stand out.
2. Second paragraph: I gave a list of reasons why I wanted to attend the school. I gave personal (parents are here), to professional (it's a ranked school with good residency match list) and financial (it's cheaper than a lotta other schools!).
3. Concluded with the 'intent to come if you accept me' and a general thanks remarks.
It wasn't too long, less than a page, and I filled it up with legitimate reasons I wanted to attend the school, showing that I researched the school (my knowledge of the match list and other misc things I liked about the school), while not sounding like I'm sucking up (didn't say 'this school is the best school in the whole wide world!!!') and I ended with the statement that I would come if accepted.