You want to keep bending her till she stops then instantly release. Keep your weight to the outside in the bend, run your hand down the inside rein, bring the rein to your hip and flex her till her nose touches your foot. The instant she stops release her. Be aware the greater the speed you are going the larger the circle you should start with, slowing to a small circle as the horse slows, otherwise you will pull the horse off balance and possibly fall over.
You might want to start on the ground, getting her to flex. I just learned how to do it from the ground
There is another thread with photos and a link to teaching the one rein stop http://www.horseforum.com/viewtopic....249&highlight=
You might give it a read.
Ps sorry I refer to all horses as "she"