Bribery will get you everywhere.... I had bought a monster truck for Mateo and upon seeing it (and wanting it) I told him that after he "pooped" in the potty - he could have it. So he went into the bathroom, de-diapered, and went poop in the potty. He was elated to have his monster truck.
But not elated enough for a repeat performance. Two more monster trucks lay sitting on the counter...waiting for Mateo to want them bad enough. Sigh. Fingers crossed we will be having another party in the potty soon.