Here is a fantastic blogpost by Andreh Anderson, a BJJ black belt under Rey Diogo. Andreh is a very cool dude, and this post is what I have been preaching to whoever is willing to listen since I have been back from Brazil four years ago. Very well articulated.

Cecil says check it out:

http://andrehbjj.blogspot.com/2009/07/sparring-time-in-brazil.html