Hey... yea hill's help A LOT but if they aren't availible...
*try getting her to use her butt more. You can do this by when you are trotting (for example) ask her to slow down (almost to a walk) with your body and hands, but ask her to keep her butt engaged by either tapping her with your feet and ' or with a whip (lightly). By doing this, you are asking her to step under herself more (aka tracking up), which definetly will get her butt working!!!
*an easy way at awalk to ask them to track up more is going rythmically 'bump bump bump' with your legs, alternating. You want to 'bump' her while her hind leg is just leaving the ground, but half halt with your seat so she doesn't speed up. For example, while her outside back leg is coming up, (when the outside front is going back I believe- someone correct me if im wrong) bump her w/ your outside leg, then same with the left
*caverlettis- very good for butt muscle no matter what your discipline is (and works on back muscles)
Umm that's it for now.. haha sorry if I don't make sense but I tried lol good luck! Hope I helped