The art of listening

What does it take to listen?
It has been my experience that few people give their full attention in conversation.
Is it a symptom of our society's increasing "me first" mentality, our too busy lifestyles or just typical human selfishness that has existed since the day that piece of fruit was consumed?

I am referring to the art of listening to someone as they speak in conversation. 
Perhaps its a friend or acquaintance that just needs an ear to which to vent about something.
Perhaps its a dialog with both parties contributing.

I have been in groups and/or business type meetings where seemingly everyone feels the need to express their thoughts, with everyone talking over everyone else.  Any meaningful input soon becomes lost in the din of noise, like so many whirring tires on a busy highway.  In those times, I sit back, observing, listening, perhaps catching a salient point.  But usually the result is a whole lot of words get spoken, with almost none of them worth the air to express them.

So, do you listen when engaged in conversation? Or do you semi-listen, thinking of what you're going to say next?