Noses and mouths are a lot more complex than I thought they were. Interesting.
References from Drawing: A Complete Guide by Giovanni Civardi.
Tips from the book concerning drawing mouths...
When drawing the lips make sure that, above all, you carefully draw the line which separates them - ensure that it lies on the semi-cylindrical surface of the jaw bones and follows the rules of perspective I have already mentioned.
Notice, for instance, that the upper lip is usually thinner and more protruding than the lower.
Tips from the book concerning drawing noses...
The nose is rather difficult to represent as it sticks out of the face and therefore its appearance varies depending on the viewpoint. Its pyramid-like shape is partially due to two small, close together bones and partially to cartilages, and this can be seen clearly on its dorsum.
Notice that the dorsum moves away from the bridge to reach maximum projection at the tip and its sides slope towards the cheeks. The triangular base hosts the nostrils, oval-shaped and slightly converging towards the tip, and delimited by the alae of the nose.
Try to work out the most important areas of light and shadow (the maximum amount of light is usually on the dorsum and the tip, while the most intense shadow is at the base, near the nostrils) and indicate just those, to avoid making the drawing too 'heavy'.
"I am always doing what I cannot do yet, in order to learn how to do it."
-- Vincent van Gogh