I work with a horse like this. He came from an abusive home and has scars on his back from an ill fitting saddle. In my situation my horse was just so -worried- and -expectant- of pain that he over reacted, it was purely physcological. What I started doing with him was grooming his girth, touching it gently with my hands and talking to him. After a while of doing this, I would set the saddle on his back and prepare for him to paw, correct his pawing with a firm "NO". Once he simmered down, I would gently hold the girth so it was touching him, but not actually tie it up to the saddle, I just wanted him to feel it on his belly and realize there was no pain. Through out this process I talked to him the -entire- time. Gradually I started tightening the girth, and now he has very minimal reaction to it, and when he does, I can simply say "NO" and he relaxes.
With a different horse (who was not as severe) I would put the bridle on first, his bit had spinners on it and it was enough of a distraction that he didn't pay much attention to the girth :)