One thing I struggle with and would like help with to improve is my line weight and variation. I've heard various artist's mention the importance of varying line width in drawings, and I love seeing it in drawings of others. However I don't know the "rules" of when to vary my line thickness especially in the outline. In a lot of my drawings I tend to outline too much (see the attached portrait of a guy for example) which tends to give it a stylized look. The other drawing of a horse is from today where I try to vary the line thickness of the outline, but I'm mostly randomly varying the line thickness. For the core-shadows I think I know how it works (sharp plain change= thinner core shadow, gradual plain change=thicker/softer core shadow). Does anybody want to share their tips, or point me to a good resource on this? Thanks