I know everyone has their own methods...but this worked for me:
1. Open up every door the trailer has so as much light enters as possible
2. Attach a lunge line to the halter
3. Don't fidgit around - when you are ready to walk up to the trailer, don't be holding this or that, securing this door or another; be prepared to *attempt* to walk right onto the trailer.
4. Encourage them to walk on with you as far as they will go
5. Once they reach their stopping point, let them stop --- but don't let them move backward. **if they try to move back, continue to encourage them to move forward until you are back at the original stopping point. This could go on for a while - so be prepared :)
6. If your horse has stood in that spot for 1-2 mins without attempting to back up, ask for another step. You will probably have to repeat step #5.
7. Once you get another step out of him, let him stand there for 1-2 minutes, rub him - then back him off (but make sure that it is your idea!)
8. Walk far enough away from the trailer to where he won't catch himself on any doors or ramps, and let him graze for about 30 seconds.
9. Repeat steps 1-8 until your horse is all the way in and standing for 1-2 minutes.
After my horse was comfortable being in the trailer, I began asking him to load where I stand at the back of the trailer and encourage him to go forward; we haven't perfected it quite yet, but we are almost there - good luck!
PS - I don't give treats for getting on the trailer - - I tried that about 10 years ago with him, and he would snatch the treat and backout quicker than he did before every time! (Same reason I don't feed grain (take a bite, back out, take a bite, back out)
Edited for : Spelling