Re: How do you keep dry flies, well, dry?
Here are some of my thoughts.
1. If you catch fish on the fly it will get waterlogged being under the water (in the fish's mouth). So while the shake stuff will help it probably won't get all the water out of the fly so expect to use it frequently.
2. Pre-treat the flies - Enolaeagle suggested doing it at home which is great, you can also do it on the water with a gel type stuff that's probably sold near the shake powder. If your fly has CDC (your duck butt feather) on it don't use the gel on the feather, but you can rub it into dubbing, the chenille tail, etc.
3. I've found some cheaper hackled dries don't float as well. Not sure if they use a poorer (cheaper) hackle to begin with or if they use less turns or what but if you buy flies it is something to watch out for.
4. I found an 'm-fly' online and it doesn't look like a real long floater. Not saying it isn't a good fly. If I found the right fly, the chenille tail will probably absorb water, the hackle isn't stiff, and while CDC can float it wouldn't overcome soggy chenille. Might be worth getting more foam beetles and spiders or some heavily hackled flies like parachute adams or wullf style if they fish will take them.
5. Also, if your fly drags (moving water it is more of an issue but on still water you might drag it when making the next cast) it can get soggy.
Probably more info than you wanted but those are my thoughts based on what you wrote!