Here at the farm we use a variety of obstacles to help our horses gain confidence and to expose them to as many objects as possible. They are also a very useful way to simulate what the horse will encounter out on the trail, such as streams, bridges, and fallen trees.
Desensitization to such things as flapping tarps and plastic bags is critical to a safe trail riding experience. We also have our horses walk through a collection of empty plastic bottles, cans, and coffee canisters--debris that sadly are all too often found on trails. We even have an old mattress (and we all know those discarded mattresses are terrifying)!
Additionally, with imagination and only a little effort, you can construct obstacles that resemble the confines of a trailer--thereby laying the groundwork and gaining your horse's trust when you ask him to go through or step onto something.
When introducing your horse to these things, it is important to use approach and retreat. Ask the horse to walk by the object at first, in both directions. Start from farther away and gradually approach the new object. When he seems comfortable doing that, allow him to approach and investigate if he wants. Reward the slightest try by walking away, then approaching again. If he is trying to do as you ask, don't push him to try harder; wait, retreat if necessary, then approach again. Think of it this way: if you were afraid of heights, and someone forced you to climb to the top of a ladder, this would only serve to worsen your fear and anxiety (and you'd probably start to dislike that person). However, if someone coached you to just begin with one step and progress in stages as you felt more comfortable, you'd begin to conquer your fear much more effectively. The same approach works wonders with horses.
It is important to watch for signs that the horse is letting go of tension when in this process. Things such as blinking, licking, chewing, sighing, or shaking the head and neck indicate the horse is coming down off adrenaline. These signs of relaxation may be obvious or very subtle, and you may have to wait quite a bit of time before you see them.
Waiting for these signs is critical because a horse cannot learn when he is in a fearful, reactive state of mind. If you put enough pressure on a horse then, you might force his body to move but he would not be thinking or accepting, only reacting---which may cause him to be worse next time. But if you give him enough "soak time," you will teach him to think through "scary" situations and to trust that what you are asking of him is not going to hurt him. This is the beginning of that special bond we all desire to have with our horses.