Apologies for this relative necro. As I was reading some Pavel the other day an article in "Beyond Bodybuilding" reminded me of this thread (which I guess I've read recently). Everyone has offered neck strengthening advice and technical advice, i'm also aware that McClaw feels a little better about things now than when this thread was started in 2010(i think?).
I'll quote this Pavel article from the aforementioned book because it appears to focus on different aspects than those already mentioned in this thread. In my edition of Beyond Bodybuilding the following quote starts on pg. 212 in the section for neck and shoulder strengthening.
"To give you an idea of sport specific neck training, here is an old school boxing drill I learned from boxing coach Steve Baccari,RKC.
Lie on your stomach; place your hands under your jaw. Steve showed me how he makes a fist with one hand and sets it down on the ground, thumb up. then he wraps the fist with his other hand. What you get is a comfortable cup to fit your jaw in. Have a training partner place his hands on top of your head and slowly apply pressure. Slowly open and close your mouth. This will strengthen your jaw muscles and help prevent your lower jaw from slamming into the base of your skull. Which is what happens when you get hit on the button, as they say in the sport."
Hopefully my shitty typing didn't mess up that quote too bad. I don't train boxing of claim to be familiar with the described drill. I personally find this Pavel article a little hard to picture like some of Pavel's other stuff. My point is that in the Pavel source they focus on jaw strength rather than the neck strength that has already been mentioned.