Slowly get the horse use to it. Don't rush it!
I bought a pair of small battery powered clippers from walgreens (super unobstrusive and quiet). Every day, I would put my filly who was terrified of clippers on the wash rack and rub the clippers all over her while I was brushing her. I also gave her lots of treats. Then when she got comfortable with clippers while they were off, I turned them on and started to slowly clip a little hair on her neck, then moved to the jaw, then finally the whiskers. Once she realized it wouldn't hurt her she let me clip her. I did this over several months, and I made sure I did it every time I pulled her out of the stall. I would clip her even if there was nothing to clip! Now she lets me clip her ears and everything with the normal horse clippers. Just take your time!
I second Andrew Davis' answer!!!Good Luck!
I would turn the clippers on when I fed grain! Everytime my horses heard the clippers they associate it with food. It quickly changed there reaction to clippers.
Let them sniff it and after the sniff it rub in on them all over their body. After you have done that turn it on and let them listen to the sound for a few mintues.
I have done some desensitization with him, using the technique of showing him a scary object (clippers) and letting him back away from it and me while I follow him. When he stops I hide the clippers behind my back to reward him. Consider that he interprets this game as 'now I'm supposed to act scared' when she shows me something.
I got my horse when he was 7 years old and he had never seen clippers in his life so he was terrified. I trace clipped him for the first few winters I had him and for a few years, I had the vet come out and give him a nice cocktail. Never did I ever twitch him to do his head. I did as much as I could and each time I was able to do more. Now I can body clip him with no drugs whatsoever. I can also do his face and head. I think the cocktails along with not doing anything drastic like twitching really helped him realize he wasn't going to die :)