Clinton anderson's passanger lessons are great for horses like this. We had some bums at the barn I volunteered at and it really did wonders. It also breaks them of the habit of wanting to stop at the gate.
Lazy horses are my favorites to work with. They're lazy and sometimes pushy yes, but the plus side off their hard headedness is that not only does pressure (squeezing, clicking, kicking) not affect them as much, but neither does scarry objects. They learn to relax in new situations alot easier, and they're good problem solvers, which is why it's always good to mix up their schedule on a regular basis, or else they'll use the routine against you ( "ahem, miss, yes you with the reins, you do realize that at the end of our cantering sessions we are supposed back up five steps and then break for lunch, right? That's how it always happens around here, silly girl, and I don't want to be late. Here, maybe you understand the cro-hopping, pinned ears and tail swishing.... )
So anyways, the passanger lessons are simple. Squeeze (gently), cluck, spank.
The squeeze is our signal, the cluck is our warning, and the spank is our reprimand. I never kick horses anymore. Not because I think it's cruel and that people who kick precious horses are terrible people, lol, but because I find it less effective. Not only does it effect our seat, but it causes the horse to be tensein the shoulders and midsection. Kinda cuts through the flow of things.
So squeeze (gently), cluck, and spank (on the shoulder or bum depending on where your horse reacts better, and preferably with something that makes a slapping noise. The point is not to beat them into it, but instead bug the heck out of them until they give to the pressure, and maybe give them a little sting of shame.) as soon as they break into a canter, reward by releasing all aspects of pressure.
The other aspect of this exercise, is to do it on a loose rein and let the horse go wherever it wants. This is what cures the stickiness at certain areas of the arena. If the horse wants to be at the gate, fine. But it will offer no rest to its weary soul. If they want to canter circles at the gate, fine, let them, and as soon as they break away from the gate, let them rest. This will teach the horse to be responsible for its own feet and stay at its cruise setting. And don't nag. If the horse feels like it's going to break to a trot, let them, and -then- correct them for it. Let them commit to the mistake. Eventually your horse will learn that the only release of pressure it will receive will be when it's going the desired speed. And then all is happy with the world.
This exercise is also great for horses who want to go too fast. Most horses go fast because they believe that the faster they go, the quicker they'll be done. Make their idea turn against them. "Want to canter a blazin hundred miles an hour? fine, I can do this all day. And I will! Boy, my horse has great ideas!" Canter them until they don't want to canter anymore and then canter them some more. After a few days of that, your horse will start to pace itself. "Gee, going fast isn't such a good idea, because who knows how long I could be doing this for?"
This method worked great for a lazy girl I used to work with. She went from a circle of teeth jarring trotting followed by a blast off canter to cantering gently off a gentle squeeze, maintaining pace, and gained a -much- slower canter. She also lost interest in the gate and stopped getting stuck in the middle of the arena. Once she no longer had an agenda, she was more responsive and stayed more attentive. She also got some pretty little muscles!