I am starting to carry neck knives more often (when legal to carry a fixed blade).
I have found that anything hanging from my neck must be extremely lightweight (no more than 3 oz. including the sheath) to keep from moving like a pendulum, thereby printing through the shirt, when I walk. Thinness is also critical, as is keeping the overall length short while still retaining some usability.
Right now, I use a Smith & Wesson neck knife. The length is a good compromise between usability and concealment, and it is lightweight enough to avoid swinging back and forth. I used the shell of a shoelace in a manner similar to the way others have used paracord, with one difference: only that portion of the ball chain in close proximity to the sheath is covered, with the majority left exposed. This way, the ball chain doesn't make noise when rubbing against the sheath, but the ball chain will still break if needed. I like the idea of bare ball chain because, in my opinion, it will look less out of place than paracord if it becomes visible outside the shirt collar. I have ordered a LaGriffe from Emerson, and suspect that this will work even better.
My biggest concern with neck knives is visibility of the chain/cord around the collar. The VanCook sheath sold by Survival Sheath Systems, mentioned in a previous post, looks like it would solve this problem and create another: someone putting a friendly hand on your shoulder or around your back would feel the harness.
Drawing is easy, even with a tucked-in shirt. Just grab the shirt in both hands and pull upward, then reach for the blade. The neck knife is a last-ditch backup. Use a pocket folder when you need to use a knife as a tool, which will attract less negative attention.
I have also heard of some carrying a neck knife by wrapping the cord or chain around the belt, and then shoving the sheath into the waistband. When the knife is drawn, the sheath comes out of the waistband but stops when it reaches the end of the unwrapped cord, allowing the knife to leave the sheath at this point.