Why on earth did I not try this before?

Scene: Yesterday, getting into the car to go to school. I open the passenger door to put something on the seat. Mabel takes her chance to jump in the "wrong" way.

Me: Don't... oh, okay, fine, just don't squash my stuff in the bag there.

Mabel notices the bag and purposely smushes a fist down on top of the carefully ironed and folded clothes I'm bringing to the consignment store.

Me: Sigh.

Mabel switches direction and heads for the driver's seat instead of her own.

Me: [lightbulb moment] Mabel, don't sit in your seat.

Mabel looks at me, confused.

Me, more clearly: Mabel, whatever you do, you're not allowed to sit in your carseat. Don't come over here.

Mabel gives me a tiny grin and clambers very purposefully towards her seat. She sits down beautifully.

Me, doing up her straps: Don't sit here. You'd better not sit here.

...

We get to school and Mabel shows signs of reluctance.

Me, wondering if it can possibly keep working: You're not to go to school today. You'll be in big trouble if you go into that classroom.

She gets out of the car, eager to break a rule, and heads for the building, and down the corridor, having the time of her life as I call after her.

Me: Don't go inside. You'd better not go through that door...

And so on. Her courage failed her when it was time for me to leave, and I did have to read a story and then have her teacher peel her off me and distract her with snacktime plans (muffins!), but it worked amazingly well. 

Just something to keep in my arsenal and pull out only ocassionally, I think, in the hopes that it might save us all a meltdown some other day.

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(I can't find a video clip, but here's the audio for exactly what this calls to mind, just to save anyone from having to go and look for it.)