I just had this same issue with two of my new horses.
Starting out I take them to the trailer and just let them check it out for a few minutes.
Then I ask them to load, the general reaction I get is some sort of pulling/rearing.
As soon as they pull I quickly back them up, take them 10 or 15 feet away from the trailer and lunge them for about a minute, sometimes more if they're especially squirelly.
Then I take them over and stop them in front of the trailer, pet them and talk to them for a few seconds, then I get in the trailer and ask the horse to move forward, if he refuses to move don't give him more than 10 seconds, then back him up and lunge again. If he pulls against you immedietly back him up and lunge.
Repeat as many times as necessary. Most of the horses I've used this method on pick up on it after only a few cycles of lunging. The general idea is to make their time away from the trailer uncomfortable, and their time near/in the trailer comfortable, so they start to associate the trailer with rest, and rest = good.
Ohh sorry, that was kind of long and drawn out! Hope it helps though, Good Luck! :)