the trick with the nylon is how you trim it. dont cut it straight, at the end of the fly. cut it at an angle, giveing a taper to the fly.
I use thinning sheers when I cut synthetic hair. It gives the fibers a raggy, uneven "natural" appearance. The best results I've got are from inexpensive coarse thinning sheers, not the fine fly tying versions. I got mine at Wal-Mart.