Determine how many statements you will need to write and how they differ. Your personal statement is not a one-size-fits-all document. Many graduate school applicants will be applying to more than one school; if this is true for you, then you must know the format, word length and general requirements for the personal statements of each your applications. Failure to follow basic statement guidelines will make you look unprofessional from the outset.
Develop a clear and precise theme or thesis. Personal statements tend to be fairly short documents -- admissions officers have to read lots of them -- so there is no space to discuss everything from your first bicycle to your last award. Clearly outline what you intend to discuss in your statement and your aim in writing it, and do so at the beginning of the piece. Admissions officers and faculty want to know, right away, who you are and what you will be discussing.
Demonstrate why you belong. Demonstrate to the reader that you are a good fit for the program. Depending on space, try to focus on your academic credentials, interests and goals. Also, highlight your strengths, if they are relevant to the program. If you are a diligent worker and clear communicator, mention this. The best statements will link these skills to program expectations -- for instance, finishing on time -- and then explain how they will help you in your research.
Mention names and interests. Once you've established who you are and why you belong, note what interests you and who in the faculty is relevant, or necessary, to your research interests. Ideally you will already have some idea of the faculty member or members with whom you would like to work. If not, research faculty members to determine who best fits your interests, contact them to see if they are interested in working with you and then note all of this in the statement. This shows initiative and demonstrates to admissions officers how good a fit you are for the program.
Edit and proofread. Once you are finished writing the substantive part of the statement, edit your work. Have a friend or colleague read the statement and give you feedback. Read the piece aloud to catch any awkward or unclear parts. Imagine yourself as an admissions officer or faculty member -- would you want this applicant in the program? Ensure that your writing is clear, precise, and formal, but not wordy or jargon-laden. Remember that those reviewing your application will be reading many statements and may not be familiar with your research interests.