One of the most disconcerting issues is a "spooky" horse. Oftentimes, the horse jumps to the side, and in some cases, the horse gets very low, turns in the opposite direction, and runs as fast as is possible to escape the enemy. Unfortunately for the majority of the situations, the "enemy" happens to be a plastic bag, tarp, blowing saddle blanket, a tree, a rock.... Being able to read the horse's body language can help you determine whether or not the horse will spook or not- ears forward, muscles taunt, blowing out, stepping to the side or stepping high- all of these should alert you to a potentially hazardous situation.
If you are already in a situation in which the horse is spooking, there are only a few options. First, you must try to get the horse to calm down for a moment. Horses have only a few options available to them- run away or attack- and most choose to run away. Once you have the horse under control, you have a decision to make: if you are on the horse, you have the option of either trying to desensitize your horse to the issue, or to try to move on past it as safely as possible. If you choose to work on the issue, ensure that you are not in a dangerous situation (i.e., a single-track trail, around a number of novice riders, etc.) and are prepared in case the horse tries to spook again. Using both legs, encourage the horse to approach the problem, using a calm voice to reassure your mount. The horse may stretch his neck out and blow out at whatever is scary, and may try to move away or spin- again, try to get the horse to look at the object until he chews a little in submission, then move away. It is best not to keep working a horse on an issue once he submits- usually this is when people push their horses too much and the horse's brain becomes fried. If you are not in the situation in which you can work on the issue, either due to safety issues, your own fear, or time constraints, look FORWARD. Do not look at the scary object, but pretend it doesn't exist. Take up both reins and give both a light squeeze (do not give them a death grip, however- keep the bit soft in the horse's mouth or you will encounter other issues, such as rearing, etc.). Again, encourage the horse by squeezing with both legs like you're squeezing a tube of toothpaste and try to move on past the object, calmly reassuring the horse with a low, calm voice. If the horse refuses, walking backwards past the object is sometimes and option. If the horse refuses to move or is getting to be dangerous, it might be necessary to get off and walk your horse past the object. Whether or not you should get off the horse is up to you, the rider. Only you know your own riding ability and your horse.
If you are walking a horse and it spooks, you also have a few options. Again, get the horse to calm down, either by turning it away from the scary object, or moving far enough away that the object does not pose a threat. Stand to the side of your horse, making sure that the lead rope is not wrapped around any part of your hand/fingers so that, even if your horse pulls back suddenly, you are not going to lose a limb. Wearing gloves is also recommended as nylon can easily burn flesh if pulled quickly enough. Allow your horse to approach the situation slowly and keep talking to him. Your horse will probably stretch his neck out and blow at the scary object, so be ready in case he pulls back or tries to bolt- if he tries to bolt, turn him around to again face the scary object. If he bows his head or licks his lips in submission, that should be the lesson for the day- walk him past the object and work on it some more the next day. "Sacking out" a horse in one day more often results in a crazy horse as opposed to one with a decent mind. If you are not in a situation in which you can work on the issue, look straight ahead and walk forward. Keep the horse occupied with you- tug lightly on the lead rope, and if he gets past you, stop him (circle him around if necessary), and make him go backwards. He should think that the scariest thing should be YOU (not because you're scary, but because he needs to listen to you first and foremost). If you have to continue to stop and take a few steps to back up, then go forward a few steps, stopping to back up, etc. then so be it. At that point, your horse is concentrating on YOU, not the object.
The best way to deal with a spooky horse is to try to work on issues in a safe environment before you encounter them. If the object is something like a plastic bag, you can work on desensitizing him to it at home- put carrots or other goodies in a bag in his stall or paddock so he will view them as a treat source instead of a threat. With a halter and lead rope on, ball up the bag in your hand and rub it on the horse while it is still in your hand- your horse might not react much, or he might act as if you are burning him with a hot poker- be calm and reassuring and work slowly. If you get five seconds of your horse standing still one day, work for ten the next. Eventually you should be rubbing the bag completely open on the horse without an issue. This may take minutes, hours, or weeks, but it can be done. If you encounter something on trail, in the arena, at a show or on the street that your horse is spooking at, then you need to be sure to deal with that problem at home so it won't be one next time.