Here's how I train a stop:
First a little background. I have two verbal commands I use: "Whoa" or "Ho", and "Hup". I use either, depending on what's going on. If I'm trying to get a horse to stand, I tend to use "whoa/ho". When I'm in the saddle moving and want a stop, I use "hup". Don't know why, just my style, but the horse recognizes both. I am presupposing that ground work has been done and your horse understands how to go, stop, w/t/c/, etc.
Start out at a walk. When you are prepared to stop, say "Hup!", then, within about 1/2 second, sit hard in the saddle, feet a little forward to brace you against the stop, and snug up the reins to give a little pressure on the bit. At first you may need a little extra pressure on the bit, which is why I normally do this training with a bosal first. You don't really need to pull the reins, just sit hard in the saddle and your body will naturally lean back a little. This should provide sufficient "pull" on the reins to give a good signal. Giving the verbal signal before the seat and reins, in my experience, gives the horse a "heads-up" as to what's coming and helps him comply better with my body cues.
At first the horse will probably take a few steps before stopping. As he does so, increase the pressure on the reins to get the stop and continue to bring them back a little more to require a couple steps backwards. As soon as he takes the steps backwards, release the reins, sit up, and praise the horse with a pat or rub on the neck. Once he stops taking extra steps, you release the pressure as soon as he stops solidly.
Do that until the horse stops without taking any extra steps. It should be a solid stop and might even include a step back, since that is what you were asking for. Once he's doing that at a walk, you can start at trot, and so forth.
It doesn't take long before the horse figures it out and begins to dig his back heels in and start the beginnings of a "sliding stop." As the horse progresses, you will eventually find he responds to your verbal command and seat and your signal with the reins becomes much more subtle.
Hope that helps.
Last edited by thenrie; 12-23-2012 at 10:34 PM.