I just found a video of a girl who's drinking a ton of water for the express purpose of filling up her bladder so she can take a huge piss.
Here it is:
http://www30.zippyshare.com/v/5OPF0pB0/file.html
(the piss is included as well and happens at 6:30)
Be warned it is a Pisswh*reTraining video
Anyone out there have more like this?