When I first started biking I didnt wear a helmet. Now I commute everyday in SF and wear a mtb helmet everytime I hop on my bike. To be honest I dont like wearing a helmet, but just yesterday I went face first over my handlebars and flipped over compleatly... I didnt hit my head but it showed me that despite how carful you are you can still easily get into an accident.

It is amazing how many people I see not wearing a helmet in this city, especially with how dangerous it is to ride anywhere in it. I have almost been hit by a car more times than I can count.

Stop thinking about it and wear a helmet before you are unable to think about it.