How we taught Chevy to stop is the same way we taught him to stop breaking out of the walk and going into the trot endlessly.
What we did is simply taught him to enjoy it, that walk/halt was a break and a treat.
The walk, we would work him pretty hard, then ask him to walk. As soon as he did it, we'd drop the reins and the contact and let him just walk around with lots of pets, scritches etc. He soon began to figure out that walk meant he did a good job and the work would stop if he did it.
Once he had that breakthrough, the walk became a refuge and a comfy place to be so he was more than happy to do it on a very light cue. Once we reintroduced work into walking (collection, lateral movements etc), as long as when he did a good job we dropped all contact and let him have an break he still took it as a good thing.
As for the halt, it used to suck. He'd barge right through it, especially on the longe. The last time I was at a longing clinic it was with my now current instructor. She saw him do it, and stopped the clinic and had me repeat. She then commented that this was a horse who'd never been taught that whoa is a good thing, a treat and a chance to stand around, smell the roses and take a break.
So we on the longe with a voice cue. We'd longe him in the end of the arena so we had walls on three sides (works with just one) but does require effective communication between you and the horse to work as it's a body language cue.
I'd ask for a whoa when he was at one of the walls, if he didn't halt immediately I stepped over a couple steps towards the wall in front of him basically 'crowding' him between me and the wall. This usually stops him immediately.
Then I'd walk up to him, scratch him and let him stop working and stand there. No work, pets and attention. After a few minutes, I'd go back to longing him. Do it again, stop all work, walk up to him and scritch and kiss and cuddle him. Soon I stopped walking up to him to teach him to stand where he is at all times wether or not I move around or come up to him.
He started consistantly stopping, letting out a sigh and relaxing. So we introduced it under saddle, using the voice cue. And that's where we are now
He's still not 100%, but he consistantly will do canter to halt on the voice cue and just stand there.
So the way we did it wasn't by training that a cue meant what (he knew damn well), we taught him that that cue means work stops and he can do nothing. And my horse is a serious fan of doing nothing!