If you could ride her I would say lots of "long and low"! Long rein, riding her from behind letting her stretch over her topline with her head and neck stretching low and out....if you can't ride I would suggest ground driving and aiming for a similar shape- if she is underweight and you are worried about riding her...lunging isn't much easier on them. It would be beneficial to get on and have very short, easy rides with the horse in a good "shape" (ei long and low to build topline) vs lunging her in circles with her body all willy nilly.
Best of luck!
Ya im not going to lunge her, was thinking about ground driving. im working on training her to flex her neck like a dressage horse like i read in another article she has had very little training and the training she had before wasnt the right kind so now she keeps her head up all the time and is extremely head shy to the point of where sometimes it takes an hour to ease a halter on but its slowly getting better. The first time i tried to put a halter on her she knocked me out by swinging her face towards me. The people said she was fully broke and was a great horse but it was a lie. although she is becomming a great horse now :)
If your horse thinks your the greatest person in the world dont seek a second opinion.