oh, fun fun.
we have a few horses at my barn that like to do the nipping thing. what i tend to do is reprimand them in a reasonably gruff tone of voice (tell them "NO!" or the like). generally, most of the horses that exhibit this nasty behavior (that i've come in contact with, anyway) will stop if you reprimand them accordingly, and let 'em know you mean business. don't be shy; biting and nudging is entirely unacceptable. then again, i've also had sucess with responding to mildly-jerkish behavior (head-tossing and pawing while grooming on cross-ties) with calming the horse down with soft pets behind the ears, speaking in a very low, soothing voice, and perhaps "shhh-ing" a bit. it all depends on the horse and the situation.
however... if i were you, what i would do first is inquire about the horse in question with your instructor or another staffmember at your barn. people at Chrislar have always been sure to tell me beforehand about a particular horse's bad habits in regards to nipping and such, and instruct me as to appropriate responses to each.
oh, another thing... if it's the girth that the horse gets upset about, you might try being veeeerrryyy gentle, and tightening it gradually. only go to the first hole at first, then continue with some other stuff before you tighten again, to the next hole. then perhaps put his bridle on, and go out into the ring. ask someone to hold him and speak to him softly while you sneak in another hole or two. then go the last bit after you've mounted, if you feel comfortable with that.
just my $0.02; if anything i've said seems wrong, i hope someone will correct me!
-cyn