[Dixie]

My family breed like rabbits and when I lived closer to them and babysat regularly, we used poster charts and their reward for using the potty (correctly) was a sticker of their choice on the poster next to their name.
After they accumulated so many stickers (say 30 stickers) they'd get an ice cream cone or some kind of tangible prize.
When you stop bribing them to learn to use the potty, you bribe them to learn something else useful. Tying shoes, reading, ABCs, fetching slippers, etc.