I agree with what Lori said, but will add to remember to use the outside rein in the bending and don't "throw him away" on that side. The inside rein controls the direction of the bend, the outside rein controls the size. A bend isn't proper if the horse is falling toward the direction of the bend, which is what will happen if the outside rein isn't used. A dead giveaway is if the withers are upright during the bend. If they tip like a motorcycle through a corner, the horse is not bending correctly, as the inside hind leg is not coming up underneath the horse to support itself in the turn. It will take some time for the horse to figure out how to keep themselves and the vehicle upright.
We went through these same exercises and goals last year with my show gelding. He had "nice" gaits, but I'm aiming for "brilliant". Two years ago, he was pretty much consistently nosed out except during the slow trot, where I was asking for more "compaction". Last winter, he was EXTREMELY heavy in the bridle, so much so that my hands would hurt after working him. It was NO FUN! Then, when we started the work that Lori describes (he just wasn't ready for it before then), he finally began to "use himself" better. Small circles did wonders. We have a small indoor, and I would have him do a circle in each corner of the arena around a cone. We would also work on Cones work, taking smaller "routes" than we did the summer before. This kept him mentally engaged (our horses get bored going round and round, but think that driving Cones is a game). By last spring, he was MUCH lighter in the bridle. I found out that this can be a relatively normal occurance with driving horses, to get heavier before getting lighter, as described in the Heike Bean book, Carriage Driving, a Logical Approach Through Dressage Training. (Of course, I bought the book AFTER we already went through the icky part of his training! I literally couldn't stand driving him, but hoped there was light at the end of the tunnel. There was. Phew!) Our other horses didn't go through the "heavy stage", but I don't think that we were asking as much either, just satisfied to go from Point A to Point B without looking like anything, so they figured it out on their own, and they had much lighter mouths to start with.
Another technique for hind end engagement is hill work. This forces them to use their butt. When they figure out how to "push" the cart instead of pull it, they will use their rear ends more. If you don't have any hills (or they are covered with snow. We are supposed to get about a foot today!
) you can add a little extra weight (within reason, they shouldn't look like they are struggling to pull the cart), it will encourage them to use their butt. If I put my powerhouse gelding in a cart that is too light now, he "slops" around and totally loses frame.
Now my gelding holds his head consistantly perpendicular to the ground, but hopefully this year, we can start to "lift" his head and shoulders more. His front end is free and he has good "strike", but he is definitely in a "Training Level frame". We see moments of an Intermediate frame, but he needs to get more consistent with that now. He is now a joy to drive.
I would forego the shortcut gadgets, they might produce a "show ring" style horse in a shorter period of time, but if you want a horse that will actually use himself properly, push from behind, track up properly, and round his frame, there are no short cuts. Personally, I think that the gadgets actually set horses back in training by producing hollow horses. Roundness comes from work, not gadgets. Eventually, when they figure out that it is easier to push the cart than pull it, they will use themselves more and the extension will come.
Let us know how it is going!
Myrna