T.W.
I know this sounds mean, however, it was the only that worked for me when my son was 3 1/2 and still using the potty. First let me say we tried rewarding him with his favorite Thomas toys and that worked for a while then that wore off!! I actually had to give him about a 6 week advanced warning that if he was not using the potty he would NOT be going on his fieldtrip with this pre-school classmates. Needless to say he awoke that morning and refused to use the potty so he did not go on the trip. I stayed home from work that morning and took him to pre-school AFTER they all got back.
He was upset,as you can imagine, but has never worn diapers since then. My doctor told me there are very few things can control: eating, sleeping and potty training. They will hold on to that control for as long as they can!
Hope this helps and doesn't sound too mean.