Good quality feathers
The following advice on tying herl is from Noel Jetson's book.
Herls close to the eye are most suitable for dry flies. The selection should be made according to the size of the fly. You do not want too much flue on a small fly. Notice that there is less flue on those herls near the eye of the moon feather and more flue on those further away. Try to select herl that is green. Tie in the single strand of herl by the butt of the herl not by the tip. Notice that those herls from the left-hand side of the moon feather (shiny side of the feather facing you) have flue on the right edge of the herl and those from the right-hand side of the feather have flue on the left-hand edge of the herl. This means that herls from the left-hand side of the feather need to be wound onto the hook in a clockwise direction and those from the right-hand side need to be wound on in an anti-clockwise direction. Notice also that you need to tie the herl onto the hook so that the flue that you are intending will form the body is at right angles to the shank of the hook. The dull side of the herl faces the hook eye. Wind on the herl so that each turn of herl slightly overlaps the other, resulting in the flue (which is on one edge only) being as close as possible to the flue produced by the previous turn.