100% of the time.
I never did when I was younger. Then, my sister rolled over her toyota three times off the freeway, trying to take a quick peek in the back seat for a CD. Car was totalled; she was wearing her belt and came out without a scratch.
Made a believer out of me. That, and reading the words of a highway patrolman in an article on safety, who said that he had never unbuckled a dead man. The implication was that he had seen a few bodies that had flown out of cars, and not survived. Buckling up gives you a better chance of survival.