Another good line is "never do anything to your helmet that you wouldn't do to your head." Always reminding the lesson kids and their parents not to chuck the helmet on the mudroom floor when they get home, or leave it freezing/baking under the rear windshield of the car, either.
Helmet Nazi? Meh, I've been called worse. Read the heartbreaking first three months of Courtney King-Dye's blog from the time of her injury. You'll see her go from remaining hopeful that she'll keep the ride on some of her competition horses and NOT being convinced that all riders should wear helmets....to reluctantly accepting when those owners hired other riders, and finally embracing her unique position to be a powerful advocate for approved helmets every time, every horse.