How can our internal/external communications profit from more knowledge sharing?

"Good external communication starts with good internal communication."

This starts to be acknowledged by more and more colleagues. But good internal communication needs in the first place  that useful information is known by the people that need it for their communication task. In many organisations this information is not shared because the person who has the information thinks it is not useful for communication, has not the reflex to pass it on, doesn't see it as his/her task and responsibility to pass it on. How can we promote this communication reflex? What is needed for all people to share knowledge better - and not only for communication purposes but also to improve the quality and efficiency of our work?