This is one of the few situations in when I count with 2.5 yo dd. Carseats are a non-negotiable. When it becomes clear that she's not going to be cooperative even after our car seat ritual (silly stuff like "here's your kiss"), I say,"OK, I don't want to count, but I can't see any other way to do this right now. I'm going to count down from 5 and then help you get in the car seat. I still hope you'll do it on your own though." Pause. Count down slowly. Usually she's in position by four. It took a few times "helping" though until she understood I was serious. If I have to help, I don't remind her about it later... so each time getting in the car I hold the fresh expectation that she will get in the seat herself.