Here is something some might find useful.
If you have trouble getting values, highs and depths right on drawings you can practice this. Draw a face from reference and draw lines into it. The lines should be like a 3D pattern mesh and emphazise the human form.
My mesh is not perfect or anything, but really liked this exercise. I guess one can do it on photos too. Like taking moms beauty journal and go liney on it. xD
It also works well to get that sense of depth in digital paintings.