We had almost finished getting our 4 yr old twin girls potty trained, except one of them refused to poop in the potty. She'd pretend to be trying very hard to poop there, but then announce that she couldn't - and then demand a diaper. Literally 10 seconds later, the diaper would be full of poop that it would be our joy to wipe off her butt. Not.
We tried bribes, everything we could think of. Then on Father's Day at a church BBQ party, we had no access to a diaper, and she'd been eating a lot of fruit that day...well, I was too busy at the face-painting booth, so my husband got to do the honors. The girl required a full hose-down, as the diarrhea had been rather explosive.
After that, my husband, my mother and I all had a brainstorming session to try and get the girl to be diaper-free, and my mother came up with the idea of getting one of those kiddie-type laptop computers with a few games on it to use as a bribe.
It didn't work at first, but after the girl saw her sister getting all the computer time for successful potty poops, she started doing it too! After about a week of this, she was diaper-free!