How would you handle a six-year-old who refuses to buckle her seat belt until asked several times?

I dont have this particular issue. I suppose Id say, "We cant leave until your buckled in, buckle it now" When it doesn't happen" Okay, when we get home, no computer(whatever) or when we get to the park, you have to sit for 10 minutes and watch your brother play before you can" Still doesn't happen? "I am coming to buckle you in" as I buckle her, "Now, you dont get to play at the park at all (or there is no tv for today or tomorrow).