Right - I did a quick search on the web & added in some stuff so here are a few ideas to get started. Let me know what stuff I've missed.....
1. RELAXED HORSE WITHOUT THE CLIPPERS
First, make sure you can touch your horse all over. If your horse doesn't like his ears being rubbed by your hand, or his feet being picked up, then you'd want to fix these things before you bring a clippers into the equation. So get to the stage where you can touch, rub you horse all over with energy, while he is standing on a loose rein (i.e. he's not physically being forced to stand still) and there are no issues.
Remember that you must treat each side/eye as an individual horse, so do all of this from both sides.
Take the time it takes. This might take a few sessions. Think 5 or 10 mins and then finish on a good note & try again later.
2. RELAXED HORSE WITH THE NOISE OF THE CLIPPERS ONLY
Have someone holding your horse, on a loose rein, in a safe enclosed arena. The basic principle is approach and retreat, you start far enough away (is your horse freaks out currently, this could be really far away) with the clippers that the horse doesn't react when you switch them on and off. You then move a little closer so they're paying attention but not too worried (if they are freaking out you're going too fast, move away from the horse). Then you just stand there switching them on and off till the horse is thoroughly bored. Every time horse does something good (looks at clippers, takes step towards clippers, put head down, relaxes) turn off clippers. If this takes time then it takes time. Don't be in a rush.
If you only have a plug in clippers, maybe buy a cheap battery powered electric toothbrush which would have a similiar sound and you can then move it around more freely.
3. RELAXED HORSE BEING NEAR THE NOISY CLIPPERS
Then you move closer and repeat, always switching them off and maybe stepping away when the horse is calm and tolerating them, and never closing in so quickly that you start to invoke a flight reaction. Sometimes the horse will become curious and move towards them, in which case make them move away or turn them off, this confirms that they're not dangerous.
4. RELAXED HORSE WITH THE CLIPPERS TOUCHING HIM
Once you're by the horse (at the shoulder) with the clippers the same principal applies, you start by running them in one hand and touching with the other, then running them on the back of the hand on the horse, then against the horse but not clipping, then clipping. You'd then use approach and retreat to gradually expand the area the horse accepts the clippers running on into more ticklish and sensitive areas.
5. RELAXED HORSE WITH THE CLIPPERS TOUCHING HIM ALL OVER
A few short sessions over a couple of days seem to work better than one long one, latent learning often seems to apply. Remember. this is a schooling exercise. Your main aim is not to clip your horse. Your main aim to train your horse to be very good to clip. Thus you take a bit of extra time on this.
Next year then should be heaps better then. You train your horse today for what you want them to do tomorrow.
NOTE: If your horse is very scared of the clippers, the above, if done incorrectly can be dangerous. A horse will protect themselves using flight or fight... so if you go too fast, get to close too quick, etc you can end up kicked or with a horse who has galloped away. If you're a novice, or haven't dealt with a horse with this issue before, it would be wiser to get the help of a good horse person who has solved this issue with others horses. Anyone else any clipping ideas / tips / stories / etc?
How do you decide what time to clip your horse, and who much to clip him?
(happy halloween!)