The head should be held upright with eyes focusing down the track.  The head should be in a neutral position where there is no tension or strain felt in the neck. You can test this by slightly tilting the head back and forth and to the sides to see if there is any tension there. Position the head at the point where there is minimal tension or strain on the neck. The head should feel weightless. The face should be totally relaxed to the point where your jaws are flapping as you run. Watch the NCAA and professional sprinters on TV in slow motion to get an idea how this looks.