What I did last weekend was to sort of flip them -- hold on to the bird, grasp the head firmly but gently, and let go with a sudden motion. It will flap downward, and the head will instantly separate from the spinal cord. Then, quickly cut off the head, and it will bleed out like a fountain. Basically, this is the equivalent to hanging. I think it was as "humane" of a method as any -- it was instantaneous, a sudden loss of neural connection between brain and body, and the eyes closed instantly with absolutely no motion in the head of any kind. The body flapped and seized, of course.
And, if you want to save yourself a TON of work, just skin them, rather than plucking. Yes, you waste the skin, but in my household we very rarely eat the skin due to health concerns over fat and cholesterol. I could skin and clean 2 birds in the time it would take to just pluck one.