Has anyone got any suggestions on a clicker programme to stop a horse from blowing out when you are trying to girth them up.

Oooh what a cool question. I've never thought about it...all my guys tend to blow up a little when girthing but usually by the time I get all my stirrups set up and get to the mounting block, I can tighten the girth to where I need to.
How about watching really carefully for when she blows out and clicking and treating it and getting it on cue. So you might start to put the girth on, see she starts to 'enlarge', c/t and then say the word 'Blow' or something...??
Then hang around until you see some relaxation or the girth getting looser..??... and c/t for that and say 'breath'...???
So then you have two new behaviours...breath in, and breath out...??...so Blow and Breathe...??
If the blowing out is just a mental defense, the playing with c/t while girthing might help with attitude...??...if it's a physical issue then I guess I would try and track that down. Maybe a bit of a sore back or sometimes I've heard the muscles under the girth just behind the elbow can be tight and that can cause some defensiveness with the girth...??
Interesting though. I'd love you to keep us posted if you give it a go!