Gridwork, such as gymnastics! And for sure trot poles!
I like backing up and rollbacks down the fence. Transitions, collection aND hill work are also great
Trot poles and up/downhill work!
Turn on the forehand and rollbacks help as well as hills and circle work
Lots of transitions, and working on collection/extension within the gaits, gymnastics and jumping, hill work, and cavalettis
Extended trots, hill work, and practicing transitions should help
Trotting up gentle hills and slowely walking down hills. I especially find riding downhill to help. It also helps a lot with balance and coordination.
Just about any exercise that allows you to get them pushing forward to the bridle, reaching underneath themselves and lifting their backs. I find circles, shallow loop cerpintines, and changes of bend work great for this purpose. Also changes of gait withing the gait (collect/extend/working)
Walking and backing up hills