The non-breathing head position is critical, and not just for the sake of reducing drag. If the head is optimally positioned and motionless, other skills will be easier to evaluate and control. The head position should remain fixed throughout the non-breathing stroke cycle and there should be no vertical, lateral, or rotational movement. One of the many misconceptions about swimming technique is that the head must be submerged for the legs to remain high in the water. Although lowering the head may help to raise the legs, breathing may then require excessive head motion that increases drag. Since the spine is closer than the head to the legs, arching the back is a much more effective way to control leg position.
Even elite swimmers have minor head motion during their non-breathing stroke cycles. Very often, head rotation will synchronise with torso rotation, and it is also common to synchronize downward head motion with the downward arm entry. Whilst these are both natural, they are also counterproductive. Although a slight amount of head motion will not have a major impact on swimming speed, it is vital to stabailise it as much as possible because of the impact on tracking other skills. A swimmer can be much more certain of what they see and feel, if their head is motionless.
Below, you will find cues for the optimal head position during your full stroke (whilst not breathing). It is important to note, that the optimal position of the water level on your forehead, primarily depends upon two factors - body composition and swim speed. Swimmers with a low percentage of body fat / high percentage of lean muscle, or a slower swim speed, may need to lower their head slightly so that the water level is slightly above the hair line. Swimmers with a higher percentage of body fat / low percentage of lean muscle, or a faster swim speed, may need to raise their head so that the water level is slightly below their hair line.
Whilst it may seem counter-intuitive for someone carrying a higher percentage of fat, to require the same head position adjustment as someone who is a faster swimmer - this is certainly the case. Fat is less dense than water, so will naturally elevate your body position. If you consider yourself to be an 'overweight' swimmer who suffers from a low body position - this is due to your slow swim speed (typically caused by an ineffective kick) or high percentage of lean muscle - rather than a high percentage of body-fat.
Look forward at a 45 degree angle, so that both the wall at end of pool and the bottom of pool are within your view.
Feel the water level around your hairline (or for masters swimmers - at least where that hairline once was :).