Flexibility is improved by stretching.
Here is an article http://www.naturalnews.com/010528.html
Which has this quote:
"Physical impact and weight-bearing exercise stimulates bone formation. Just as a muscle gets stronger and bigger the more you use it, a bone becomes stronger and denser when you regularly place demands upon it.
The best bone builders are exercises that put force on the bone, such as weight-bearing activities like running and resistance exercises like strength training. In general, the greater the impact involved, the more it strengthens the bones."
So, you need to train those ribs and collar bones to take some punishment by basically getting beat on regularly.
There was also a special on Discovery Channel in 2006 call Fight Science which was interesting in that it measured the strength and power of a fighters body. They may have mentioned the bone density thing in that special as well.
Personally, I prefer to watch MMA/UFC and if I was going to fight, that's what I'd rather do. However, I'm at least a bit vain in that I don't want my nose broken or facial scars and I need glasses - so I'm lasik and a motor cycle helmet away from UFC training.