Here's a swell pencil drawing of Ernie by Jim Smith. Jim is very good at suggesting form even with rough sketches. If the drawings aren't carefully cleaned up or inked, they can easily flatten out and lose their impact. So it's important to analyze a drawing before you start inking. The first thing is to note how the biggest forms are constructed and how they relate to each other in dimension and position.
It's also a very good practice to connect the cranium to the body with the neck - even when you don't see the neck because it is behind the chin.
A lot of artists have problems understanding the relation between the cheeks and the smile lines. the line at the top of a cheek and the line underneath that describes the smile are 2 borders of a piece of meat that you use to make expressions. They should look like they make a form that points to the nostril.