Transitions transitions transitions. Challenge yourself to ride 50 transitions (or more) a day. Set a goal and COUNT them. On my big, strong, leaning horse I may do a transition (up or down) every time I feel him lose balance, lose focus, or lose impulsion. That may be after 5 strides, or 3 strides, or 10 strides. Transitions within gaits also count towards your total. Transitions between 2 gaits (trot/halt, walk/canter, canter/halt) count double (+2). Every time the horse feels heavy, use your seat, shoulder, leg, and finally hand to ask for a downward transition.
Make every transition the best you can. Don’t worry about where his head is, or how soft his mouth is in the beginning. Just ride it well: straight, no pulling, no leaning. Good body control from YOU, as well-- don’t forget to let go! Don’t forget to use leg, especially in downwards! Ask his hind legs to step under and lift the shoulder in every transition. It will be ugly in the beginning, and that’s fine. Just keep trying, and insist that the transitions keep happening with a minimum improvement each time.
Doesn’t matter where you are in the arena; transition. On circles, on centerlines, on diagonal lines, on serpentines, in corners. Sometimes pick a point to perform the transition, and work on setting him up for it. Other times, ride the transition when his balance calls for it-- RIGHT NOW, not 3 strides later when he’s even heavier on your hands.
I find that after the first 25 transitions, things get easier. The horse starts to realize…hey, I may need to pay attention today because AT ANY MOMENT I may need to use my butt, sit down and halt (or walk, or canter). They start to listen to your seat, and your half-halt starts to be “heard” and respected. The heavy ones get lighter. The hot ones calm down. The lazy ones get more responsive and lighter to the leg. Correct transitions really can fix just about any balance problem, and over time they will increase strength, too.
After you are bored to tears from transitions, work on poles. Karen O’Connor used to ask her students to canter a line of 6 bounce poles. Novice/Training level horses set at 9ft apart (standard, ordinary distance). Preliminary horses needed to shorten to 8ft apart. Intermediate event horses set at 7ft apart. And Advanced eventers set at 6ft apart-- requiring a VERY collected, light, bouncy canter, and self-control (from horse and rider!) is mandatory. If they rush at all, lose balance, they will mess their way through the poles. If you have the ground help, start at 9ft apart and roll them in 6 inches or a foot as the horse does them successfully.