We all as a family sat down a picked a "major prize" for 2 weeks of no accidents. We figured 2 weeks of success was a successful toilet train.
DD1 picked going on a airplane to visit her aunt for a few days while dad was down there on business. Then flying back with her grandparents who were coming up for a visit. Worked like a charm.
DD2 picked a Bob the Builder bicycle she'd been eye-ing off for months. It too worked brilliantly.