Shimazaki makes the best stuff in the world for keeping your dry fly on top of the water. It's evening, that dry fly you're using is perfect, the trout can't say no, but it is soaked from one too many takes and won't float, unless you apply this stuff. I use the spray bottle and/or the dry shake. It's expensive, but really worth it when you need it most.

Apply just before walking down to the stream and you’re good to go for several casts. A quick little spritz as needed will keep your fly high and dry. Easily the easiest to use and most effective floatant I have ever seen. Expensive compared to other applications but well worth it. I had used other types and brands but have used this exclusively since discovering it.