How difficult it is depends on the model. It's easy to rig an animal model with long jaws, but hard to rig a creature with cheeks, such as a human. I sent a tutorial about it to Daz, but it's not up yet. But basically what you do is attach a bone to a few polygons behind the actual jaw, so the jaw uses that bone as a pivot point. The downside of this method is that it only works well with models that were made with the mouth open, because the jaw sometimes deforms oddly when opened further.