Trailer loading seems to be one of those things people neglect to teach horses until they get ready to go somewhere. Then, in their rush to get the horse loaded, they succeed only in creating a bad experience for the horse. This training needs to start early in life and be carried out in a calm, gradual manner.
Loading a horse onto a trailer is the ultimate test of how well you've refined all the groundwork you've done with your horse thus far. If you have done a thorough job of building trust with your horse, trailer loading should go pretty smoothly.
When loading young horses or any horse for the first time, it's wise to have someone around in case you get in trouble. You want someone there in case of a problem, but you should be the one to get your horse into the trailer. Make sure the person is patient and respects your approach to handling your animals.
Make sure the trailer you use is exceedingly safe. There must not be any sharp protrusions or odd places for a lead rope to get caught. It shouldn't make much of a difference if the trailer has a ramp or is a step-up type, although most horses seem to negotiate the ramp style better.
Before loading up, put a halter and lead rope on your horse. Put a bag of hay in the trailer. This is not so much as a bribe to get her in but a comfort item to greet her after she loads and to keep her busy during the trip. If the hay bag offers one more incentive for her to climb in, great, but you should be careful not to entice her onto the trailer physically before she's mentally ready to be there. The idea is to build trust so that the horse follows you up the ramp and loads willingly.
The way to do this is to lead your horse to the trailer and let him check out things. Have the gate open and let him look and smell inside. As long as his attention is on the trailer, let him investigate. If you sense that he is becoming interested in things other than the trailer, do what it takes to return his attention back inside. Send some energy up the lead rope or turn him around and lead him back to the entrance if you have to.
Horses are careful about where they put their feet, so that first step on the ramp will be scary to your horse because it feels different from standing on firm ground. Be calm and encouraging, but don't rush. Give him some time to get accustomed to this new experience, one step at a time.
If the trailer is big enough for more than one horse, it sometimes helps to load an experienced pasture pal to show the newbie that there's nothing to it. The last thing you want to do is frighten and betray the horse; if you do, you will probably always have problems loading.