J.M.
One thing we tried that I think came from Oprah was to buy panties with a favorite character on them and tell her that the character got sad if she peed on them. It was also mentioned that the character (aka gramdma or a friend) could call them and give positive reinforcement.
The main thing is to just keep taking them to the potty. Sometimes i made my daughter sit there until she peed. If it's a battle of wills...that is what is going to have to happen.