J.C.
With my kids I made a big deal of pulling off into a parking lot or something and refused to go anywhere til they buckled up. Helps if you're going to ice cream store, etc.
The other strategy I've used is saying that if they don't buckle up we'll have to go to the fire station and ask them what they think about the child being out of his/her seat. This prospect always scared my kids into complying. I even pulled into the parking lot of the fire station before.
Since the firefighters are willing to install the car seats, I think they would be willing to circle the wagons and talk to a child about staying in their seats.