At a certain point I realized you can get much more arm movement into your sketchbook drawing. This was difficult to accomplish for me because I was so used to drawing mainly with my wrist and I would choke up and grip my pen really hard. One of the reasons I'm able to get many different looking lines is because each line represents a different part of my arm I'm moving. For small hatch lines, I'll move just my fingers. For bigger lines like a jaw line I'll move my wrist but to get the line of someones back down to their feet, I'll use my entire arm.