I live in Australia, and it's pretty much second nature for me to put on my seatbelt whenever I get into a car. Partly because I drive an old car that doesn't have airbags, ABS, traction control or much in the way of safety at all, and partly because the fine for not wearing one is so massive I can't afford to pay it.
Also, I've crashed a car before, and my seatbelt certainly saved my life then. I hit a post at 80 Km/h and walked away with a slightly sore knee as it bumped steering column.
The only time I don't wear a seatbelt is when I'm in my brother's 1955 Austin A50, and that's only because it doesn't have seatbelts. It was built before they became mandatory.