"In just 10 seconds you could save a life. It won't even cost you a dime. This act of self sacrifice won't earn you a parade, it won't make you popular, it won't land a date with... you know who, and it definitely won't win any fashion awards--but it just might buy you enough time do everything else. Wear a helmet and the life you save is your own.
It took ___ months in the hospital but I am now able to hear out of one ear and I might be able to ______, but the doctors aren't sure. I'm one of the lucky ones. Change your life forever, wear a helmet. It's worth the hassle.
---Scroll List of people killed without helmets---" End.
In the end though I think nothing beats a personal story being told by the people affected. Just put it in their heads that it's not worth the risk. It's ultimately game theory. "Helmet is guaranteed to be annoying. Danger to self is remote and abstract." Make the danger real, close and relatable.