A common mistake students make in personal statements is believing that admissions committees will be convinced that they deserve a place in a program just because it had been a childhood dream of theirs, or because their parents worked in those professions. Although childhood dreams or parental influence can begin a compelling narrative about your motivation to pursue a career path, admissions committees like to see that initial interest translated into active commitment to career-related activities. The key to using a motivation from childhood is to make it very brief, and make sure it flows with the rest of the essay.
Students often make the mistake of going into lengthy elaborations of research projects in highly-technical language. Essays dominated by highly-technical language can alienate readers that are outside their field of your specialization and make the writer seem like he has difficulty communicating appropriately. Also, the admissions committee has to read dozens of essays and might put aside an essay that is difficult to read or understand in favor of one that is not.
Students often rush through the personal statement and submit their first or second drafts, but in order to produce an excellent statement, an average of five or more drafts is advisable. It's a good idea to get feedback on your personal statements from people outside your immediate family and friends. Seek advice from instructors in a writing center in your school; they have experience helping with personal statements. If you don't have access to a writing center, a student or professor in the program or school you are applying to can usually be quite helpful.
An admissions committee is interested in understanding the student beneath the surface. Therefore, simply stating that you are interested in a career, or you have certain qualities will not suffice. When writing, use precise examples to point out the qualities you possess and also to show your committment to a specific career path.