I have done the delphinuim corsage and boutonniere quite often. One thing I have always done is dip the bloom in either QuickDip, or in a solution of white carpenter's glue and water. I make sure to keep a little fan near by so after I dip, I give it a little shake, and then in front of the fan (gentle fan of course - or you could blow on it) to make sure the delicate petals don't glue together. Then I let dry on a sheet of waxpaper.
I do the same thing when I wire delphinium for bridal bouquets.
Even in really hot and humid weather, it seems to hold up quite nicely.
Of course, before doing any of that I make sure I process the entire stem by injecting the water up the stem and then putting it in deep water for a few hours...