Horses That Won't Load...?

The lesson horse I ride is extreamly stubborn (as most school horses are) and he usually refuses to load, so we usually spen almost 45 minutes getting 3 horses loaded! Any ideas on how to get them to not be so afraid of the trailer? We have tried coaxing them in with treats, food, sweetfeed and almost everything else and we even resorted to stud chains. Once they are in they don't seem to mind, it's just getting them through the door that's the problem. There is no way for us to practice loading them on normal days because the trailer comes the moring of the show and leaves right after we get back.