Everyones ideas are very good, I've done most of them and they've worked for me (I also agree that I wouldn't force her to go potty). Right now I also have a 2yo DC boy that is like this (thank goodness he's only a drop-in) no matter how much you watch him (shadowing). I'm thinking it's because he's not here regularly so he's not in groove with us. I bought one of those child leads (the ones that velcro around their wrists) and hooked it to my hip and strapped the other end to the back of his pants
. I did it because I was at my wits end with him but it ended up being very productive.
The problem I was having was that even if I shadowed him he would run off and I would have to chase him. The lead prevented him from doing that and he was not happy. Explaining to his mother why he was tethered to my hip was a little strange but hey, when you work with kids you have to get creative.