Simple: There's nothing to the nose... just put it in the right place and watch your line weight. Get that first nose down and move on to the second... learn how to draw accurate foreshortened nostrils.
On the mouth, pay close attention to where the lines are heaviest. This is probably the most important area to control your marker well. Every little change in weight will make a different shape or expression.
Next post: A lot of what not to do