My instructor gave us this technique. Like a lot of our tricks, this one employs lateral work and bend. If I can pull this off early, I can eliminate or reduce the severity. If I miss the subtle signs, and he goes off like a cannon, I can use this technique to "bring him back".
Exercise: Bending through a spook (or shy, or bolt, or what-have-you)

2) At the same time: activate the inside hind-leg. Apply your left (inside) seat-bone (or upper-leg or calf if you don't have a strong seat aid yet.) Push the left (inside) hind-leg so that it steps underneath the horse. This inside driving aid is very important. It keeps the horse moving toward the "scary" object. Apply more aid than is normal. Your leg aid should match the degree of rein aid: more rein = more leg.
3) When you feel your horse relax, immediately relax your aids. This rewards your horse for listening to you, rather than acting on impulse.
4) Repeat as necessary. You can repeat these aids again, even a few strides later, when necessary. Just remember to relax and reward when your horse is good.
The key here is that your aids are stronger than usual. I only employ the strong aids for enough time to get him obedient and then I relax and reward.
You might also want to check out Jane Savoie's write-up that sounds like the same idea.