we have a two year old 
gelding paint that we are working with. he was imprinted at birth and 
has been worked with a lot.  currently the trainer working with him 
is "sacking him out", so to speak, and she doesnt do it in his stall. 
 what were doing is this:  mostly in the indoor arena, at the end of 
a lead line, and we give him time to investigate.  now image isnt 
exactly the bravest horse in the world, but he is very curious and 
doesnt panic.  i rememebr when he was about 6 months old and he was 
terribly hung up in his water bucket.  he got stuck, made a little 
bit of a squirm, and when he found he couldnt escape, he started 
whinnying for help.  when we got in the stall with him, he jsut stood 
there and let us untangle him..  i think one of the big things with 
this "sacking out" biz is that the horse is imprinted from birth.  
all of our are (wish everyone did this), and each of our horses knows 
that we would NEVER allow anything to hurt him/her;  they do trust us 
an awful lot.
mary and one smart poky dot