Pretty much every topic regarding motivations for becoming a doctor will be cliche. When you have 40,000 essays every single year, people are going to eventually exhaust all uniqueness. Don't worry about being especially unique, just be honest and sincere, with a good tone. Also, don't be writing as if your personal statement will result in some sort of epiphany for the reader (hard to describe, but you might know where I'm going with this), for the reason I stated above.
Some cliches:
Dualistic aspects of humanism and science
Epiphanic experience
The "getting my act together" story
The gradual development of interest through activities X,Y,Z
etc.
It's all been done. So don't worry about being unique, just be yourself. The REALLY important thing is a good tone and your spin on stuff. The goal isn't for the reader to say "Wow I never thought of that before." The goal is to first interest the reader (maybe with a descriptive introduction of a particular scene or theme), and second to leave an impression similar to "Wow this is a good person who matches the personality I'm looking for in a physician."
That's my take on the whole PS thangy thang. That's just my opinion so take it with a grain of salt. Cheers!