You simply teach them to load and unload themselves. Walk the horse up to the trailer and send them forward. It might take some training, but eventually they get the idea that they're supposed to walk forward.
Some people put the horse on a lunge line and put it through the sidedoor so they can pull on it and the horse feels someone tugging him forward and he'll walk right in. (Most people can just put the leadrope on the neck and let the horse go right in). Once he's in (have the chest bar up), bolt the buttbar and you can put the ramp up if you want. Then walk over and tie the horse up for the trip.
Once you're there, untie the horse, throw the rope over his neck, take the ramp down, take the buttbar down, and let the horse back out. Just grab the lead rope off his back and help him down. Simple.
If your horse refuses to load without help, walk him in, keep the leadrope with you as you walk out the sidedoor, then drop it and put the bar/ramp up.