First, you should take heart in knowing that no matter how problematic your personal statement is, it is probably not as bad as Bonnie's was. However, you should keep your eye out for the issues that came up in Bonnie's essay.
First, remember that the essay is a "personal statement." The admissions committee is looking for information about you that they cannot find elsewhere in your application - information that reveals something compelling about who you are as a person. Your essay should be focused and coherent; it should have a kind of "thesis" that will persuade an admissions committee to see you as an unique individual who has qualities that will enrich their graduate population.
Your writing should be clear, vivid, and concise; it should also strike the appropriate tone. Avoid colloquial language and cliched expressions (remember: you want to sound thoughtful and unique). Also, skip the self-indulgent and/or "unattractive" stories about yourself. If you need help in identifying such stories, seek it out!
Above all, start writing early enough to allow time for revision. Personal statements are, perhaps, one of the most difficult essays to write; you will not get it perfect on the first try. Have other people read your essay and give you feedback. You can get friends to read your essay, or you can take your personal statement to the Composition Center or to the Academic Skills Center. Also, you will want to read the advice in Writing for a Job or Professional Audience, on our Materials for Students site. Finally, Carl Thum, the Director of the Academic Skills Center, offers each term a one-hour workshop on personal statements that you might want to attend.