This has worked for me: I recommend working the your horse in a round pen, about 60 feet in diameter
with this exercise:
Make a surcingle out of a long soft cotton lead rope and a cinch.
Then make a breast collar of sorts out of baling twine. Hook the
breast collar to the center dee on the cinch, and to the buckles on
either side. Don't hook it to the lead rope. Then run twine from
each side of the breast collar up over the horse's neck in front of
the withers to keep it from slipping down. (This will look like an
english breast collar). Once you have this in place, work the horse
around the round pen while you stand in the middle of the pen
holding the end of the lead rope surcingle. Tighten and loosen the
"cinch" by pulling on the lead rope. This will put you safely out
of biting range, and allow you to de-sensitize the mare by cinching
and un-cinching her many, many times.
For more training q + a, go to
www.horsetrainingquestions.com