The Sketchy Sketchbook of ....Sketchyness!!

[neopatogen]

Lots of work here, awesome sketchbook man!
Did you try using less soft brushwork? I think it would benefit your portraits, cause that much soft brush makes it look plastic.
I'm also into Proko studies btw, you seem to be ahead of me :)
As for shading simple shapes, seems like there's not enough contrast even for matte surface, did you check out Scott Robertson's How to render book? It's a great help for rendering simple forms in my opinion.