Begin with a period of self-assessment. Identify those specific experiences that have influenced your decision to attend graduate school, such as classes, lab experience, internships, volunteer work, work with a professor, personal experiences, and summer jobs. You want to write about experiences that make you unique and separate you from the rest of the candidates.
Look at sample personal statements. There are two books in the Office of Academic Advisement's library, Writing Winning Personal Statements and Graduate Admissions Essays: What Works, What Doesn't, and Why.
Always answer the questions asked. Some schools will ask very general questions, others will ask more specific ones (e.g., discuss experiences outside of the classroom that have influenced your decision to attend graduate school).
Avoid writing a résumé in prose (a narrative version of your résumé). You might want to include a résumé as part of your application, but you certainly do not need to list everything you have ever done. Your personal statement is not the place for it. Remember that it is an essay. Your personal statement should have a clearly defined theme and focus. There should be transitions between different parts of the essay and there should be an introduction and conclusion. Your personal statement is not an informal chat, a letter, or a résumé; it is an essay.