Applicants should treat their personal history statements as a story--that is, to link all of the information they provide into one overall narrative. How did your early education (elementary school, high school) affect your choice of a college and the path you chose to take there? Was there a particular person (a teacher, a relative) who affected your decisions? How has your cultural background played a role in your research interests and pursuits? By making your life into a story and finding the unifying themes, you make it easier for the admissions committee to see you as a potentially valuable member of their community.
Similarly, how has your background affected the skills and talents you have naturally and those that you have chosen to develop? What extracurricular activities did you pursue in college, and how did those work together with your academic pursuits to create the person that you are today? Make sure to put these skills and talents within the context of the larger narrative; it's not important that you're a champion ping-pong player unless you can make the argument that this has been meaningful to your academic or personal background.
Make sure to put your personal narrative in the context of the school's programs. After all, the admissions committee has to be convinced that this program makes sense for you, personally, as well as for the academic community at their school. Discuss the elements of the program that contribute to areas of knowledge you have already developed; if there is an aspect of the program that addresses something you haven't yet studied in detail, emphasize why you are interested in pursuing it based on your past interests and current skills.
Finally, finish your personal history statement with information about where you'd like to be in the future and how you see the program contributing to your professional goals. Don't be too vague; you need to make the case that this particular program can help you get to where you want to be. Conversely, don't be too specific; if you want to be in a tenure-track position at Harvard University in a particular department within the next five years, the committee might doubt your understanding of how the hiring process works. Try to strike a balance, again putting your future goals in the context of your past achievements.