I have always been told to pull the horse's head to either one side or the other - it doesn't matter which - then ride the horse into an ever decreasing circle until he eventually stops. Once he stops, either get off, or take a few minutes for both of your to calm and settle down, then begin at a walk - walking all the way home if you have to.
I did this once on a woods trail, where two trails intersected, which gave me quite a wide area in which to turn. I think the important part is getting that head around and holding it there - he can't run very well in this position and will eventually circle, slowing down and stopping.
The pulley rein is pretty effective, especially if you can't safely turn (ie narrow trail - and some horses can get too unbalanced). For the pulley rein you shorten one rein up, so your hand is about half way up the neck and brace that hand over the crest and then pull back on the other rein. Kinda hard to describe but hopefully that makes some sense.
I think it is important to keep in mind that a runaway event can happen to anyone that is riding a unfamiliar horse and sometimes and old pal you’ve been riding for years. The advice above is all good. The circumstances will dictate your best options for the situation. A key concept is you must commit to yourself that your NEVER going to give up the job of driving your horse. You never just riding on the back of the horse. It can take a long series of varied efforts on your part to pull the horse back under control, but in any case don’t let the horse win.
It happens! Your riding along thinking about what ever and bam, you find yourself no longer in control. But understand that by using your brain you can deal with a runaway. Once your back under control, and you will be if you don’t give up, you can stay in control anytime you are focused on control. Do this by using the terrain as your friend. A fence is there for you to use. When you feel your on the edge of staying in control, ride along a fence line. When your horse gives you that uninvited burst of forward motion pull his head towards the fence. If your fast enough, you won’t be going a hundred mile an hour and the horse won’t see a place to run with that fence in the way. They don’t like to slam their nose into things and my experience is they stop their sullyness quick if they feel you rather have the horse hit a fence then you ride a run-a-way. It is trickier when no fence is around, but I’ve used trees as barriers as well. Getting off and walking should be your last resort, as this gives the horse a win and tends to erect a philological barrier that will be very hard for you to put behind you. Your there to drive the horse, and you can’t do that well if your scared.
Develop good habits. Never give up. Understand that sometimes you must fight the fight to get it back under control, and there is something to be said for learning how to fall and get back up. For that, I suggest learning how to snowboard.
The pulley rein or one rein stop IS very effective in this situation. I know as my arab mare bolted on me numerous time when she was young and green.
However...be VERY CAREFUL not to pull them into TOO TIGHT of a circle as it is possible to upset their balance and thus causing both of you to FALL. Another poster mentioned it as a decreasingly smaller series of circles and that's a great way to think about it.
But beyond stopping the bolter...you must do the ground work and training AT HOME as well to teach that this is NOT acceptable to prevent it from happening in the future. Watching a horses's body language even from the saddle helps...I was able to learn the body language of the "irritated ears" and feel my mare start to tense up in a certain way....and I got to where I could tell she was thinking about a bolt.
Once you get to the point you can recognize when it might happen, immediately re-direct their attention...keep them busy, busy, busy. I would stop and turn my mare around and practice backing or side passing or leg yield or something to take her mind of whatever made her want to bolt.
Also, if I knew certain situations that might cause a bolt (for example my mare was and still somewhat is spooky with cows) I'd be sure she noticed them WAY IN ADVANCE . She also used to bolt if we were riding on a trail along side a road and a motorcylce or loud vehicle came behind us...so if I heard it from afar I'd turn around and let her see the scary object head on....it's much less scary to them if they see it coming. She'd still dance and jig a bit but not bolt if she knew what the "monster" was.
And if you can't figure out the causes and how to prevent it...GET HELP NOW from a professional trainer who can help you.
I read through the replies that people posted and they had all the same message about pulling your horse's head to the side. This is correct, except one important factor was left out in all of them. Disengaging your horse's hindquarters. A horses hindquarters are their power, their driveline. A horse can still run away with you, or buck you off, even if you have your horse's head jacked around to your leg! The first thing I teach a colt is to laterally flex and yield their hindquarters. When you yield their hindquarters, you effectively take away their forward impulsion.
You need to work on this on the ground, in a snaffle bit, getting lateral flexion and softness. Pull out wide, bringing your hand and reins to the D-ring in your saddle. Don't release until your horse give's. At first your horse will probably resist. Stay in close to your horse's side, moving with it. Make sure their feet stop moving and they give in the face before you release. Release immediately
though, when they do give. Eventually, a horse will soften and become feather-light as their rewarded by your consistent release. Repeatedly work both sides.
When your confident your horse is soft, get in a controlled area, one that they cannot runoff in. When you mount, your looking for the same response you got on the ground, for them to give softly on their back without their feet moving. After they give softly repeatdly both ways, as your bring your horses nose toward your leg, move your heel behind your back cinch or toward the flank area on the same side. You might have to bump the hindquarter over at first with your heel. As soon as the hindquarter yields, release pressure with your heel, but maintain contact with the rein until the horse gives softly in the face. Do the same thing on the other side. Work both sides until your horse learns to softly yield away from your leg pressure.
The hand position you draw the rein with is extremely important. Your arm needs to be straight pulling to the inseam of your pants. If you pull up toward your waist you'll affect the horses equalibrium and possibly tip them over.
That's how they'd tip them over in the old westerns.
Secondly, this is a taught response. Since your horse has learned to runoff,
it's learned to resist or react. You need to work on the mental softness aspects. If your horse is rewarded for response it will learn to respond.Set it up for resistance to be work and response to be relief
The goal is to regain control. There are many ways open to you in this situation but you have to think fast.
1. Vibrate the inside rein and start half halting. Put the horse into shoulder fore or shoulder in.
2. Tug and release both reins and repeat. (I used to gallop TBs).
3. Horses have 1/4 mile flight distance at best (if the cause was a genuine spook). Ride it out and gradually take your control back.
4. Calm the horse and start talking to them in a calm quiet voice. Soon, they start wondering why they are running.
5. Muddy fields are great for slowing down a horse if you can find one.
6. For experts only, for the hard core horse; Make them go faster, start using your whip, keep them going until they are exhausted. I have only had to do this once and the horse will remember not to do it again.
I don't use the old pull them into a circle. I have found horses who simply keep going and end up spinning like a top!
The goal is to make them work very hard and make running away difficult for them. Horses have one track minds so the best solution is to break their train of thought and subsitute WORK for their misbehaviour.
For the really Hard Core horse the best thing to do is to get rid of them.
Mike's comment was excellent, the key is moving the hip over and someone else said keep their feet busy, keep them thinking of something else, whether going in circles or running flat out. I was at the Mane Event in Chilliwack and saw some good examples from Stacy Westfall, Craig Cameron and Doug Mills of how to teach a horse from the ground up the proper way to respond. You gotta build the brakes into your horse from the ground up. There's also a great article in October's "Perfect Horse" by John Lyons about how to stop a Runaway, in it he also talks about turning them yes but getting that hip over as well just like Mike describes.