One thing you can practice is to slow her, with your body more than your reins, to ALMOST a walk, then ask her to move on back to a big trot. Then slow to almost a walk, then back to trot. YOu try to find that edge where she is almost breaking to a walk then ask her to trot forward. If you do this enough, she will start anticipating the downward transition. When you feel her anticipate it and soften when aske only with your body, then take her all the way to a walk, and be ever so soft on the reins. When she makes a good transition to walk, give her a huge loose rein and pet her with a big Good Girl!