I just discovered a great product which does not cake and matt the fibres on your dry flies at too and penetrates the individual fibres, leaving them completely impervious to water. The product has been used for a century for the waterproofing of garments. It is called Driza-bone garment dressing.

I have just filled my empty Gehrke's Gink container with the paste at a fraction of the cost of the real thing, and I suspect it will work even better than the stuff it replaced.

I would be interested to hear what other alternatives you guys find works for you. Mario uses Scotchguard and I would imagine that that would also work very well. Anything that repels water basically, but also manages to stay on the fly long enough, without having to re-dress it after every three or four drifts.