Re: How do you keep dry flies, well, dry?
Thee short answer to your question is: you don't. Dry flies are going to absorb water and eventually the sink. especially after landing a few fish. For myself, after three or four fish, I cut the fly off, hang it on my fly patch to dry, and tie on a new one.
You've already got some good info on floatants you can try, so I'll just add the brand I use: Loon. I like Loon because it resists temperature extremes better than Gink. However, I have buddies that like Gink, and I haven't much experience with it. Except that I know in hot weather the stuff gets a bit runny.
In the end, all floatants work pretty well the same as far as I can tell. However, after a few fish, you need to do some 'maintenance,' to get the fly riding high again.
First, you need to dry the fly. This can be accomplished a number of ways: I keep a micro-fiber hand towel in my pocket, when I remember to bring it. I just drop the fly into a fold in the towel and gently squeeze out the excess water. Micro-fiber gets a lot more water out of the fly than cotton, faster. second, reapply floatant and third, get back to fishing. You can repeat this a couple times, but after a while, the fly just won't want to float. Then it's time to retire it to the fly patch for drying before it goes back in the box.
Another method is to use dry shake. This is flakes of dessicant in a small jar that you drop the fly in and shake it to absorb the water. This works a bit better than the towel and a lot better than false casting/using your fishing shirt. You can either buy dry shake or one of its competitors, or you can make your own.
However, before going to all the trouble of drying the fly and re-dressing it, you may want to see if the fly works 'as is.' Fish often will still take your dry fly even if it's sunk a couple inches below the surface. And most of the time you'll still see the swirl of the take. Sometimes they prefer the fly sunk (especially if there's some chop on the water). In fact, there are several 'drowned' fly patterns out there, such as drowned hoppers.
Just a thought. Anyway, there's my two cents.
"Three-fourths of the Earth's surface is water, and one-fourth is land. It is quite clear that the good Lord intended us to spend triple the amount of time fishing as taking care of the lawn." ~Chuck Clark