How can I draw realistic hair with pencil?
There is a natural temptation when drawing hair to think in terms of individual hairs and equate these to pencil lines. However this tends to get in the way as there is no reasonable way to draw every individual hair even in hyper-realistic styles. 

Instead forget that hair is lots of individual strands and concentrate on what it looks like as a surface/form/texture and in terms of light and shade. It is often easier to draw smooth, glossy hair as this tends to have more defined highlights. 

In this example image you can see distinct bands of light and shade which are far more prominent than individual strands. 



It may also be useful to look at ways in which hair is represented in sculpture and more abstract graphic styles such as comic strips and art nouvea. This may help you to think in terms of the form and texture rather than the material. 

also bear in mind that photographic realism is not always the goal in portraiture and often you are trying to capture an impression of a subject rather than just mechanically reproduce an image.