Our pediatrician suggested we use popcorn as an incentive.
It is an extremely sensory product. He told us to make microwave popcorn every day. DS could hear it popping, he could smell it, and he could see it when we took it out. But, popcorn is only for big boys, and big boys don't pee and poop in their diaper. They use the potty and wear underpants.
It's not important that you eat a lot of it, just a bite or two, it is just important that he know that he can't have it. We didn't say it every day, just when he asked for it.
I can tell you, the first time we went to the movies after he was trained, he was the first to remind me that he got popcorn, because he was a "big boy".