I hated wearing my helmet and never did until a few weeks ago when I go hit for the first time by a car (made out lucky). Now, I can't even imagine not wearing a helmet. I made it a few blocks away one time and turned around to go back and get it. Helmet-hair be damned!
I've made it out a few times without a helmet and rode pretty anxiously.
When I was a kid, I put my head into a metal gate, and that was the reason I did not ride for many years. When I took up riding again a few years ago, the first accident I got in involved me going over the handlebars and my helmet skidding along the wall that was next to the road (along Chicago just north of the Chicago/Evanston border).
I've had enough concussions and contusions to tempt anymore.... :)
A friend of mine was in Iraq, came back, went to school, tripped and hit his head juuuust right and is now in one of those wheelchairs with all of the weird instruments on it. he was in a coma for a year or so. Now while he wasn't biking, i mean, just take that into consideration. I can't imagine letting just one screw up/mishap/someone else's fault mess my life entirely just because i didn't wanna be a dork and not wear a helmet.
as for when i forget my helmet (which i do sometimes) i always turn back to get it. I also just leave it in a spot where I can't not pick it up on the way out.