Remember, your horse might not be built to have a lot of stocky build muscle on her. Horses are bred to have bigger looking muscles, it doesn't just happen with exercise.
Trotting up in down hills with your horses back rounded. Lots of backing!
Loping in circles and stops, roll backs.
*Side note on backing. To back a horse, you DO NOT pull back on her face. She will hollow her back, raise her head and drag her front legs backwards, thus not gaining any muscle. You need to squeeze/kick her by her girth with both legs, and teach her to back with her head down and minimal rein pressure. Just in case you didn't know:)