I have had great luck with clicker training to work through clipper issues.
I started my yearling clipping with the clicker, so she had a good experience from the get go, but I could sit on the ground while clipping her legs if I chose to, and she could care less. She also lets me clip her ears, inside and all, with no fight.
I have tried twitching in the past, but that is just a temporary solution where as you can teach them properly with the clicker. I have also gotten better results and ended up with happier horses in the end.
I started out teaching them the basics of clicker training, having them touch (or target) objects. I moved on to having them target the clippers while off, then turned them on and target, then ask them to allow me to hold the clippers on the nose, leg, etc while on, and eventually progress to actually clipping. It really doesn't take long once they realize that there are good things equated with clipping!
I like to introduce them slowly over a few days. However, I did clip my neighbor's pony one day, we had a show the next and no more time to work on it. I rushed him through the steps a bit, but he was very willing to cooperate once he realized that the clippers were a good thing. The only other time they had tried to clip him, two people had to twitch and hold him down while a third jumped in to clip. Though he was terrified when I first brought out the clippers, he was standing quietly and happily by the end.
Anyway, that's just another option to consider that has worked well for me. Good luck!