I remember when I was orienteering I read an article by Gareth Bryan-Jones in which he advocated "5-yard memory": instead of watching the ground under your next step, keep your eyes on the ground about 5 yards ahead of you. I never got further with this than making a conscious effort to do it for a while, and then forgetting and lapsing back into watching where I was going to plant my next footstep; but it certainly did improve my running over rough terrain while I was doing it. Obviously I should have practised it until it became automatic.

Although it was recommended in the context of running through rough orienteering terrain (typically forest), I think it would also apply to downhill running.