Many responses to this thread view "defensive skiing" as a mechanical or technical based issue. And in one perspective, it is. Many have discussed the defensive mechanics that are inherent to a fearful or inexperienced skier.
But in the current venacular, where we spend more time ripping ACROSS the slopes, almost as much as down them, I view it as an AWARENESS issue, (similar to driving). I teach awareness as the means to be defensive- where are the obstacles on the trail, be they people or solid, stationary objects.
Avoiding those obstacles, by understanding your movements in correlation with those moving around you, is a skill often not developed at an early stage of skier development. But it should be.
Awareness of traffic flows, the skier in front of you, etc. , offer the opportunity to anticipate and react, prior to any event.
This lack of spatial awareness is responsible for almost as many negative situations as out of control skiers.
But thats another topic...