Collection and Cavallettis
Use transitions, collection and extension in the trot and canter, cavallettis are great, and ride them "on the bit" or ride with contact. Make sure they are lifting and rounding their back.
Ground poles, raised poles, stay in frame. Lots of trotting in frame. Lots of downward transitions.. when lunging use side reins to help encourage a frame to keep the muscle and prevent the neck from becoming inverted, if applicable to horse. For example, my greenie stays in frame under saddle with contact to reinforc, but on the lunge unless I use side reins, she raises her head and her neck becomes inverted and it defeats the purpose . If its a hard keeper possibly look into temporary supplements like fat cat to encourage muscle growth. :) hope this helps
I use lots of raised trot poles! Gets them using their back more :)