Buy a pulse clock for your horse! It will help you make your training more effective and provide injures (lameness for example) caused by too hard training.
Hill work! Been working on the strength with my guy. Even if you only walk up and down a couple of times, then working up to trotting:)
In the ring, I usually do lots of trot to canter transitions and pole work. For jumping, build a gymnastics line. Also if your able to go out of the ring, lots of hill work!! Just a few suggestions there's many other ways to build up your horses strength :))