HA! We did the up& back method at my petsmart class. Jack is so flexible, he would actually follow the treat back with his head, until his head was almost lying on his back. I think what I finally had to do was push his butt down with one hand, while holding the treat in front of his face with the other. He got really good at it…now, he sits when I snap my fingers, when I say sit, or, my favorite, when I salute him.
Good luck!