Make sure your fly is dry, either straight out the fly box, or after a few false casts, then put a small amount onto your fingers and rub / massage in to your dry flies. It will float dry flies for hours, just redo when it starts sinking. It can also be used on tippets, fly line etc.
Gink is the most important product in any fly-fisherman's waistcoat. Used by more fly-fishermen than any other dressing in the world.