The first step is to approach the trailer in the correct way. If your horse is already looking past the trailer or turning away, he has no intention of going in.
Make sure you keep your horse’s focus on you and on the trailer before thinking of getting him in there.
Then you have to get control of the feet. It is important that you are able to control every step your horse takes and your horse is feeling relaxed at all steps.
Because when you have control of the feet, you can ask the horse to go in or out.
So what to do when your horse gives the wrong answer and rushes out? Make him work, so he understands that it isn’t much fun out there.
But make sure to give him rest when he comes towards the trailer again, so he understands the difference.