Instructions
Build your relationship with your horse in a controlled environment. Do groundwork in an enclosed arena so it becomes accustomed to your cues and expectations. Ride it regularly in both indoor and outdoor arenas so it is accustomed to your weight and aids. Work with it frequently, at least several times a week, to keep everything fresh in its mind. The more it trusts you, the more it'll relax when you are working it and the less likely it will be to spook.
Introduce common scary items to your horse while you both are in a controlled environment. For example, plastic bags are something horses frequently spook at. They are noisy and blow out of nowhere. Allow your horse to sniff plastic bags, rub them on its body and attach them to common areas like outside its stall or -- when it's ready -- to the saddle. Work in small steps to get it accustomed to items that really stress it out. Scaring it too much will exacerbate the problem.
Expose the horse to as many new situations as possible, and reinforce the exposure by going back to the same environments. Riding the same trails and going to the same show stables are good ways to get the horse used to new places and things. As it grows accustomed to these conditions, it will be less likely to spook.
Give your horse time when it does spook at an object. Sit on its back in a relaxed position, and allow it to stretch and snort at the object. After a few minutes, it will relax and you can move forward. Praise it for its efforts and make it walk by the object even if it is not inclined to do so. It doesn't need to get too close to the object, but it is important that it continues on.
How to Teach Your Horse to Not Spook
Horses are prey animals, so their instinct is to flee from scary or intimidating situations and objects. In horse lingo, this is called "spooking." When a horse spooks, it may jump to the side, back suddenly or make other quick and erratic movements. A spooky horse is dangerous and unpredictable. It is important to calm the horse through regular, consistent and encouraging training. Once the horse trusts the ride and is accustomed to a variety of environments, it will settle and be less likely to spook.