My son is only 2 1/2 and is doing OK, but a friend of mind told me what worked for them.

Their daughter loved Chuck E. Cheese, so they made a big boad game path with a lot of different colored spaces and Chuck E. Cheese at one end of the path and home at the other. Each time she went pee in the potty she got to move one space and poop was 2 spaces. If she had an accident she had to start at home again. When she got to the end she got to go to Chuck E. Cheese. I have heard a few other similar ideas and all of them say it only took a couple of weeks to potty train because the kids were so intent on moving their game piece forward.