My sister's DH used to never ever wear his seat belt. Nothing she could say would get him to wear it. Finally, I told her to ask him HOW ARE YOU GOING TO KEEP CONTROL OF THE CAR WHEN YOU ARE THROWN OUT OF THE DRIVERS SEAT? SO THE KIDS DIE BECAUSE YOU WON'T WEAR YOUR BELT?
She said the next time they got in the car, and he didn't put on his belt, she asked him that. He looked at her, put on his seal belt, and has used it every single time since.