RE: Grouse, What's the secret to bagging these "fast flyers"
As a previous reply noted, it takes lots of practice. But what kind of practice? Practice with the dog and the terrain and shooting. Try taking another hunter with you and position them for the shot off of the point. If you can begin to see where to position a shooter to take good shots you will soon find yourself hunting with a particular understanding of where to be in each type of cover.
Good luck!