Well with my filly, I exposed them to her every day (without touching her) for 5 or 6 weeks and she showed no signs of improvement. I've used food, praise, I've clipped my gelding in front of her pretty much every one of those days.. She went from better to worse. The first few days I could rub her on her neck/head with the clippers turned off to toward the end I couldn't even bring the clippers within sight of her without her being tense. We finally twitched her, which also didn't end well. I'm going to try a few more things, but if not.. the next time the vet comes out, I'll probably tranq her and just get it over with.
I know I'm not the most experienced horse person here, but hey anyone who wants to try to clip her.. come out here and tell me what I've been doing wrong ;].
Although, I do have to say that the first couple months I've had her, she had little to no respect for me.. only tolerance. Lately she's been in for a few surprises and is slowly learning respect. So she might be better once we set boundaries for one another. I've only had her since the beginning Janurary.
Quieter clippers I don't think will work, she doesn't like the vibration. She barely lets me touch her whiskers, much less bring any "tools" toward them. She let me cut her leg hairs with scissors, so I think she's just head shy, despite the fact that I've rubbed her head and ears consistantly for the past two months.