For a variation on Pendragon's method, for the few life-sized heads I've tried in clay, i've always taken a pipe attached to a board with a threaded flange and then bulked it up by firmly wadding up newspaper (not flat layers!!) and attaching it with masking tape.
The advantages of this particular method are 1) you don't have to hollow as much later, and 2), when the paper gets wet from the clay, you can easily modify the gross shape of the head by whacking it with your fists (very therapeutic!).
Here are two caveats:
1) the head has to be structurally supported from below, or the piece will slide down the armature and the pipe will poke out the top. That means either some part of the piece has to touch the base (for example, the bottom of the neck or shoulders) or you have to block it up with chunks of wood or pillars of clay or summat.
2) The first layer of clay has to go on slowly and dry a little so it stiffens and doesn't slide and creep off the armature.
Incidentally, the clay cone method works really well on small scale work for neck support, when you are doing small figures. When you first form the torso, you pinch up a spike of clay, and a couple hours later, while you've been working on the rest of the bod', it is stiff enough to start sticking a head on top.
I have not tried the clay cone method on a large scale, but it sounds cheap and easy to me! :)
Like Sculptor says, you have to hollow the whole business to a fairly uniform thickness before it dries. If it is difficult to reach up the neck and hollow it, slice the piece in half from top to bottom along a plane dividing the back of the jaw and the ears. Use a needle tool to probe for thickness while hollowing. Score the edges and make some clay slip to glue it back together.