Please wear a helmet whenever you ride your bike.
In college I rode my bike everywhere without a helmet - didn't even own one. There was this guy in town who walked around with a limp and his head permanently cocked to one side. Whenever he saw somebody riding without a helmet he would shout "you have to wear a helmet or you will end up like me!" or some such thing. I wasn't hearing it. He was just an eccentric street person and I was young and invincible and I didn't care. My friends and I used to joke about what a weirdo that guy was.
Now that I live in California I wear a helmet whenever I ride. It's not that I suddenly decided that the eccentric guy was right and I was wrong. I started biking with a helmet because practically everybody here does it that way. Over time, I have gradually grown to believe that the protective benefits of a helmet outweigh its costs ($40+ sticker price, minor inconvenience, helmet hair, etc.). And obviously now that I've fallen and hit my head a couple of times, I'm a true believer. As one close family member put it:
Dave would be dead or at least very badly injured if he didn't wear [a helmet] religiously.
Please wear your helmet. I know that you are "just going for a short ride" and that you "don't ride on the street" and that you "can't find your helmet sometimes" and that you "think it looks dorky" and that you "don't want to mess up your hair" and so on. Just wear your helmet. Seriously. You'll be doing me a personal favor.
Want to learn more about bicycle helmets, including tips on buying the right one? Visit bhsi.org.
Still not convinced? Then at least please consider signing your state's organ donation registry.