The first point I would like to make about this subject is that drawing people in my mind is really about drawing figures, whole people, legs, arms and bodies as well as faces or heads. I would not consider just faces on there own to be drawing people I think of them as portraits but to be honest the rules are the same for all drawings be it cars, bicycles, dogs or cats the same basic rules apply.
Doing drawings of people or rather drawing figures is just like every other type of drawing that you will ever attempt to do because as with all other drawing it is about getting the construction right, it is about getting everything in proportion to everything else.