A good way to jumpstart the writing process is to answer three questions:
1) Why do you want to become a doctor? The answer to this question typically involves a compelling incident in your life. For some individuals, it's an experience as a patient (e.g., a battle with cancer or whatever), for others it's an experience as a caregiver. In most cases, the motivation to pursue a career in medicine is a sort of calling--i.e., one can't imagine doing anything else.
2) Why do you want to become a doctor ? What features of the medical profession do you find attractive? For me, the trust at the very heart of the doctor-patient relationship is the most attractive feature of the profession. Some other "pull" factors that are commonly mentioned in essays include the transformative potential of medical practice (i.e., the opportunity to make a significant difference in the lives of others), the application of science in a human context, the challenge of lifelong learning, etc.
3) What makes you unique as an applicant? You have to assess your unique qualities in order to distinguish yourself from other applicants with similar "hard" credentials (GPA, MCAT). If you've participated in college athletics, you should focus on the tremendous dedication that attends this extracurricular activity. Many athletes train 4 hours a day, and get up at the crack of dawn to do so; this dedication is remarkable and should be addressed in any personal statement. In my case, I had a lot of teaching and tutoring experience, so I focused on this life experience in my essay; the prime reason for this focus, aside from the fact that not very many applicants have significant experience in these areas, is the fact that excellent communication skills are essential for good teaching. Likewise, the effectiveness of doctoring rests largely on the ability of physicians to communicate, both verbally and nonverbally, with their patients.
If you answer these questions, you'll be in good shape for your personal statement. And by the way, admissions committees want these questions to be answered in a personal statement.
Good luck!