Some advice I got before writing a PS:
1. Don't write about what makes a good surgeon. Coming from a student, it may seem insulting to the readers who have their own opinions and much more experience. I included two brief stories of surgeons who I looked to as role models and why I thought they were excellent. These examples were pretty standard cases where the attending went the extra mile.
2. Don't say, "I'm good with my hands." No student is technically good, and saying so would show that you don't recognize your limits.
3. Don't be weird. My PD told me that he never gave someone an interview based on a PS, but he eliminated some applicants based on their out-of-the-box statements.
4. Self-praise stinks. Your CV and letters will highlight how great you are. Use your PS to talk about your interests, motivations, why you love surgery, what you want to do with your life.
5. Don't stress about the PS too much. Spelling and grammar seem to be very important though.