Its tough if you don't have your own trailer, but the trick is to get them to think of the trailer as a safe place. Its not something to do quickly it takes time and patience. Unfortunatley most people just want to get the horse in, get to where they are going and get the horse out. Ride. Repeat.
I start mine out just getting in to eat and rest, gradually working up to going a few 100 feet moving up and down the driveway working our way up to driving down the road.
Its not the loading and unloading that's hard on them, its the trip itself. Its not fun back there for the horse, and can be terrifying for a young or inexperienced horse. I wish everyone would train this way. It would make it easier for future owners. Take a ride in the trailer with your horse. It will give you new perspective on what its like.
You know your horse will load. Next time you have a trailer available try making it a less scary experience for her. Don't get mad, that will get you no where.
This is one of those times that patience will be your best asset. Once she is in take a short trip let her out, let her eat some grass or do something pleasant, load her back in and repeat. Spend a few hours or a day doing this, I guarantee she will load in any trailer.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France