In regards to the flies, there are several theories and preferences. In my personal record "juicy", fat larvae have been performing best.
How to catch the big ones? I found most of the times the largest barbel are in the middle of the shoal, but this might be just a supposition of mine.
Explore, observe and experiment on your own, this is always the best way to learn more. Each water might have fish with different alimentary attitudes and behaviors.