It really depends on the horse.
My horse was in a trailering accident, and even before then was not a great loader. But now he hops onto the trailer so quickly I can't actually be in front of him in the trailer. He is a self loader.
What he used to do was get frantic and run back and forth in front of the trailer, rear, bolt, etc.. So I just made him stand in front of the ramp, just stand still, and then waited. Patience is key, give yourself an hour, and have hay just in the trailer (I always have at least a handful of hay in the manger when my horses get in). Eventually she should hop up. Don't punish anything, just reposition her calmly in front of the trailer if she moves.
I used this method for another horse who had not every been trailered apart from the trip to the vet check when he was purchased. We did the same thing with him. No pressure, just hop on when you're ready. He also now self loads. Takes me under 30 seconds to get both horses loaded by myself.
Don't over complicate it. It's the same as anything else. Make the right thing easy, stay calm and don't make anything more dangerous than it has to be (lunging by the trailer scares the bajeebus out of me). Patience is key. The horse did not learn how to stand in the crossties perfectly in one day, or to be in a stall, or to jump a 4' course. It will take time and good experiences.
They say money doesn't buy happiness -- well happiness doesn't buy horses!