Thanks for the heads-up, Fizzy, I'll take the clicker discussion from here.
I have brought around many horses of this sort using clicker training. Do you have a stall that she can get her head over? If so, start this out from the other side of the wall, that way you are both safe, you from her, and she from you. ;-)
There are a couple of ground rules, for safety's sake, that you need to abide by. Always click the behavior BEFORE you reach into your pocket for the treat. Always extend your arm and hand away from your body to deliver the treat. Take the treat to the horse, make sure that the treat is uncovered only when the horse's head is in the position YOU want it to be in.
As for the clicker, I don't use a mechanical one myself, unless I am working the animal at a distance. I simply make a loud click by popping my tongue off the roof of my mouth. Frees up your hands for other work.
While your mare is standing on the other side of the barrier, hold up an object that she can easily see. It may take a while, but she will eventually come over to see what the heck you are doing. Make it easy for her to bump the object with her nose. As soon as she makes contact, click at that moment, lower the object from the touching position, then deliver the treat, following the safety rules above. Now, it may take her a few minutes to come to where you are if she is really scared or for whatever reason is not interested in people. If so, we may have to go to another strategy. Once she is consistently touching the object, you can add in a verbal cue, "Touch," or "Target." Assuming that she begins to "operate the vending machine" on a consistent basis, the next step is to:
Switch whatever object she is touching to the halter. Just a halter wadded up in your hand. Do not try to halter her for the time being, just be happy if you can get her to touch it. Use the verbal cue you installed on the previous step.
Then you will want to gradually open the halter up, clicking and treating along the way. Slip it onto her nose, click/remove it from her head/treat. You will see the steps along the way that will ensure that your mare continues to have little successes, rather than big failures.
Put yourself in her horseshoes. If you were approached by someone you didn't really care for, you might try to avoid that person if you could. If that person rushed at you, you might be frightened and even be ready to hit! But if they walked toward you in a friendly manner holding out a $20 bill, well, you just might stick around to find out what they had in mind.
For more information about clicker training, please take a look at the following web sites:
http://www.theclickercenter.com
http://www.clickryder.com
http://www.equineclickertraining.com
http://www.canadianclickercentre.com
http://www.amylacy.com/fm/Clicker%20Training.pdf (I wrote this magazine article, and there are a couple of good photos on it showing the safety positions)
http://www.clickertraining.com (all species, even human!
Good luck, let me know if you need further assistance.