Our two and a half year old daughter was potty trained fairly early just before she became two. This went quite smoothly, and after a few weeks she had almost no accidents. During her afternoon naps however, she still wears a diaper and she also consistently pees, even when she went on her pottie right beforehand.
In a few weeks she will go to preschool, where she can still have her afternoon nap, but can't wear a diaper anymore. So she will have to be 'clean' also during her nap. I know for certain she is able to physically hold it up for the time her nap normally lasts. She just has to make this mental click that she shouldn't relax this during her sleep.
How can we help her do this? We've already switched back to cloth diapers and we are making her aware (just before her nap) of the fact that from now on she should be able to hold her pee up during her nap.