It'sprobably already been covered but my expierience has helped me to draw the following conclusions.

Physical fitness has a lot to do with it. A greatr fighter and close friend of mine and I had a lengthly conversation lately about shots "bouncing off you" when in peak fitness.

As does mental state and attitude.
If you don't want to get hit, when you do take a punch the pain will be amplified.

Natural pain thresholds are also a factor.
The body has a limit, there is only so much it can take before shutting down. When it comes to trauma of the cranium that limit is relatively low.

Bone structure is a factor also.
Guys with narrow necks and shoulders are more subsceptible than stockier fellas.

At the end of the day, the one way to improve your "chin" is to protect it.