1) Choose your layout strategically. I had two separate drafts in front of me this evening. The first was a statement by chronology, the second, a statement by subject. Print out a draft, cut up the paragraphs, have a seat on the living room floor and rearrange your topics until they become more adhesive. In my case, things were made much stronger by topical layout.
2) Show your SOP to your letter of recommendation writers. Your letters should fill in the blanks that your SOP doesn't expatiate. My boss, for instance, included an anecdote about my being singled out by the benefactor of one of my undergraduate research grants at the reception following my presentation (which I did not know she was attending!), and that this woman is now a friend of mine.
3) Keep the intro short and powerful. Honestly, three sentences introducing your unique passion, your resolve and perhaps a plug for the program are all you need to be epic, here.
4) Every paragraph should relate back to your research interests, experience and future. Don't talk about what a passionate, hard worker you are; show how you are and what you've done.
5) Read out loud. Seriously. All of the errors that your eyes didn't catch will be like fresh wounds to your ears.
After five drafts, I am finally proud of my self-representation. I struggled more with this essay than any other I have written in my life.