My answer to everything is more deadlifts. I also do a lot of static holds. My gym actually has one of those cervical neck machines. But I'm interested in responses to this as well.
Wear a helmet of some kind for a few years. your neck will be giant. lol I have never worked my neck out, specifically, and I'm scared of those cervical neck Machines. sorry maybe someone else can help you with that.
I have been doing a lot of deadlifting and some overhead presses for several years now and my neck has gotten much thicker from those. Recently I started doing a lot more overhead presses, so hopefully it will get even thicker -- I'm hoping my neck will be bigger around than my head eventually! Good luck and don't forget to work on shoulders and traps while you're at it . . .