I agree with the advice to go slow, it also helps to walk the horse a bit between gradual tightening. We've spent the last year really looking into this problem and have a website dedicated to it, which one might find by searching for Girth Shield or Dressage Snob. But briefly, girthiness seems to have a lot to do with how horses go about their breathing strategy. (Interesting tests in AU about this with a group of race horses some years back as well as the British tests about rib cage expansion). When horses work they use diaphragmatic breathing--essentially sloshing their guts up against the backside of lungs every stride to assist the exhale phase. When they are in the cross ties getting tacked up they are breathing muscularly and when you tighten a girth up all at once they can literally stop breathing for a moment. (You'd lay your ears back too!). There are designs of girth that help a lot and they are not what you think, but with any girth going slow, moving the horse, longing for a while will help. In out tests we found horses liked natural fiber cord girths, narrow girths by the elbows and very limited hardware low on the body. Ours did not like overly elastic models. We do our girths up quite tight--over 20 lbs of pressure and no longer have any issue.