Any drills for looking while riding? I have come to believe that the major skill when riding singletrack is scanning the terrain. After any turn there could be a hiker, a fallen log or just a boulder that you didn't memorize during the last pass. From then on, the reflexes, embedded in the muscle memory, kick in and braking/turning/dabbing/hopping occur.

I attempt to look as far forward as speed dictates, and not to get fixated on obstacles. Still, the perpetual concentration becomes tiring after a couple of hours. 

Any tips on both how to both improve scanning at high speeds and how to become less fatigued by a whole day of riding?
Just keep on doing what you're doing.

To get good at anything, the easiest way is to keep doing that thing.

As an aside, I find I descend a lot quicker when I'm wearing a cap with the peak down. I think that having the peak in my field of view forces me to keep my head up.