When my pony refuses, I ask him back to a walk (if he isn't already) walk him back to the rail, turn around, stop, take a few deep breaths, tell him it's okay (he gets REALLY nervous) and then pick up a trot and go for the jump again. If it's an in and out I'll do the whole thing again, if it's the second jump in a line I'll just turn around at the first jump.. (:
JDI's idea works, too! :)
I didn't watch the whole thing, but your flat position is pretty good.. I could never really see when you were going over the jumps while I was watching.