I haven't tried this... but I thought I would offer a different approach.
Someone on the forums, someone I respect, has used treats to get her horse used to being inched after a bad experience. Basically you cinch as usual, nice and slow and when the horse goes to nip you, they get a treat instead. So it's a nice surprise.
Another method would be to look at the cinch. Maybe it is too narrow or an abrasive material and the horse is expressing discomfort. Maybe he has a rib out and that's why he makes a face, have you had him checked out by vet/chiro/horse masseuse?
"Strength is the ability to use a muscle without tension"