Defensive driving is really just a specific application of situational awareness [0]. With practice and experience you'll go from just maintaining visibility and space for reaction time to actually remembering where drivers are that have been blocked from your view by other vehicles. You never really track every vehicle and pedestrian, but you do make note of particular ones and the rest get lumped into groups based on their behavior. You'll learn to anticipate the car that's going to make a left turn in front of you by taking your foot off the throttle earlier instead of braking at the last second.

[0] http://en.wikipedia.org/wiki/Situation_awareness