I recently stumbled across this elegant and informative explanation of trail braking here. I've been practicing this technique since I first began driving half my life ago, but would never be able to provide such simple, accurate instruction in my own words.
It's really not as difficult as you may think, and should already be somewhat intuitive if you've been honing your fast driving skills for a few years - still, this write-up and accompanying graphic help visualize what's happening in such a simple and illuminating way that it will definitely make me more aware of what I've been doing instinctually, and thus allow further refinement of my approach, and I imagine it will provide similar help to others.
'Public roads offer a perfect venue to learn the 'feel' of trail-braking, as you can develop the smooth footwork that’s required for this technique by gently lifting off the brake pedal while turning in at slow speeds. Freeway on-ramps and off-ramps are great places for this. Work on coordinating your brake release with turning input. Only release as much as you are willing to turn. Only turn as much as you are willing to release. The relationship between steering input and brake release (even throttle) needs to be directly proportional. I call this 'string theory' in that you imagine that there is a string tied to the bottom of the steering that is connected to your big toe. If you turn the wheel that string will pull your foot off the pedal and if you push your foot down on the pedal it will in turn straighten the steering wheel. When driving n a track at or near the limit of adhesion (how much grip your tires have), too much brake release results in oversteer, while not enough brake release can cause the front tires to understeer.'
'Obviously, the best place to practice trail-braking is at the track (while entering corners with plenty of run-off and no traffic around), but if you lack the 'feel' of the maneuver, the learning curve is very steep. Start at slow speeds and build up safely while below the limit.'