On day 2 of her booster seat usage, I was driving Rebecca home from Stomp. About 2 km from home I hear 'click' and look in the rearview mirror to see Rebecca scooting across the rear seat.
"Rebecca! NO! You do NOT undo your seatbelt, now I have to stop the car because that is NOT safe!"
I pulled over, got her buckled back in, and I gave her a little lecture about how if she's not buckled in, and we have an accident, she could fly around the car and get hurt. I told her how a seatbelt keeps her much safer in the car, and she should feel uncomfortable in a moving car without a seatbelt.
It must have been effective, because as soon as I was done the waterworks started. She was pretty much done crying by the time we got home, but as I opened the front door to let her in the house she let out a big wail "MOOOMMMY" and ran up to mom and gave her a big sobby hug.
We ran a few errands in the car today, and she's still excited about her booster each time she gets to ride in the car, and there have been no more unbuckling incidents. In fact she's been asking me if it's ok to undo it every time I shut off the car.
Chalk another success up to 'the stern voice'.