Chin is a funny phrase and kind of a misnomer because it gives the impression that the phenomena of your brain slamming into the walls of your skull has to do with how strong of a head you have. This doesn't really have anything to do with the way the brain moves inside of your skull.

When you talk about a good chin you're then ultimately trying to explain ways to keep your brain from jumping around. Its common in Boxing to talk about chin like its some kind of unknown magic but if you think of chin in this way you can be scientific about it and begin to find ways to improve chin (which is to say find ways to keep you're brain from slamming into your own skull).

I think you guys have already touched on those things pretty good. I think TM and Donny bring up great points. I don't think you need to have a really huge neck though. Just a good solid neck I think is good enough. Hagler never had a big neck just a good solid one. With the chin tucked its hard for your head to be swung around from punches unless they are very strong and heavy punches.

Personally I feel that just having a good awareness of your opponent is the most important contributing factor for how well a fighter can take a punch.