10. Use poor spelling or poor grammar _ This one should be pretty obvious. Academics on admissions committees are generally high achievers with high standards who won't disregard even simple typos. If your personal statement is not technically perfect, it can make you seem sloppy, lazy, or inattentive, which are not qualities anyone wants in a future colleague. Remember that the people skimming your essay are seeking a reason to reject your application and make the pile of possible admits smaller. Always get someone with strong writing skills to review your essay.
To help you begin the writing phase, it is advantageous to split the writing task into three parts: the body, the conclusion, and the introduction. You can do so chronologically or follow your own sequence depending on your personal preference and style.