Training Science Articles:
First, I want to say, stimulus control is only for a training/working situation - you can put a cue on stimulus control, but it doesn't mean the behavior won't happen on the animal's own time.
Stimulus control is a personal choice - you don't need to put a behavior on stimulus control, though I do advise it, especially with potentially dangerous behaviors. Another reason I really like to put behaviors on stimulus control is to put the animal at ease - that they don't need to be performing ALL the time, throwing behaviors at me. This is why I advise that most behaviors should be put on stimulus control. Though some people like to allow more freedom of expression within a training situation.
So what is Stimulus Control? Karen Pryor has 4 rules the behavior must meet to be considered done.
These are self explanatory, but how do we achieve this? I have two things I do regularly that maintain stimulus control with my horses.
The first sounds silly, but I reinforce my horse not doing anything! The first thing I teach them to do is to stand still and calmly face forward, not doing anything at all. My only "cue" for this is nothing - I move all around them, I do all sorts of thing - a lack of any information should mean to my horse "just relax". I build up enough duration on this to be able to walk circles around the horse or groom them or go get something quickly. But once I start teaching other behaviors staying still is less fun, it's something they have less control over - they can touch their jolly ball or toss it to get my attention. They can exaggerate their other skills to try and get my reinforcement, but they can't exaggerate doing nothing, only with more duration. So I find it vital to continue to reinforce stopping, relaxing, facing forward (a general forward, not a strict position), whenever I don't ask for something in all my training sessions I like to have a few pauses, especially when we start to get a little too excited. I want my horse to feel comfortable not to do anything - that is good too.
Another way I put a new behavior on stimulus control (also to test my cue) is by adding in well known cues to our session. This is my favorite rainy day project. The more skills they practice this with the easier it is for them to understand with newer, more complex skills. For example, the first few skills I teach are targeting and a good back up. Once they understand both behaviors and both seem to be on cue I will begin to alternate them evenly in the same session. Once I think it's solid I'll double up one cue occasionally to see if they catch the change. Be careful in this process not to be cuing one behavior while they're doing the wrong one, you don't want to accidentally mix up the cues.
Once you have a few skills on stimulus control horses tend to grasp the concept and it gets easier. I wouldn't risk teaching a potentially dangerous skill, like rearing, striking (Spanish walk), or anything like that until I have a few easy, safe skills on stimulus control, so I know how the horse handles it. Some horses are much better about this than others.