What did it for me was watching my uncle have a rather painful death in his 50s because he couldn't stop drinking and smoking. (He went into alcohol withdrawal in the hospital after lung surgery.)
That being said, I did smoke a few when I was in my early 30s. Something about nicotine just put weird thoughts in my head a few days after smoking a cigarette: One day I was biking home and the thought "it would be a good idea to have a cigarette before making dinner" popped into my head. I never touched cigarettes after that.
Cigarettes are more addictive than people who've never tried them realize. It's not just a matter of will power, something about nicotine manipulates your motivations in a very subconscious way.
But there are a couple photos of me "smoking" as I was trying to look cool.