We use a boning knife, similar to this one,
(not the same brand) but I think Del got ours in the sporting goods section at
Walmart, in the hunting area. We keep it razor sharp, and it works great. Keeps a good edge.
We use the Chef's Choice electric sharpener that both Scubaforlife and Petrelline mentioned, and use a good chef's steel from Chicago cutlery for touch ups. You can slice right under the jaw, where the feathers are absent or very thin, or if there are some feathers there, sort of part them and get the blade on the skin, before slicing. Sawing through feathers will dull your knife in a hurry.
To remove the head, after the bird is bled out, I cut only the soft tissue all the way around, then twist to break the neck, and twist the head right off, so I don't dull my knife on the neckbones. You can turn the knife so the dull side is toward the bone, and stick the knife in alongside the bone, cutting the tissue
outward, away from the bone. The closer to the head you cut, the easier it is to remove. The further down the neck you get, the bigger the vertebrae, and harder to break. The connection between skull and neck is a natural fragile point, so it's easier to remove the head at that point.