shoomlah:

hylianjean:

anatoref:

Light and Shading Tutorial

This breaks it down a bit more toward the level I need

good tutorials are few and far between ( a lot of artists seem to get caught up in the superficial trappings of portraiture rather than the nitty gritty fundamentals), but these breakdowns by Bryan Lee are just fantastic.  Super applicable tips and tricks, no matter your personal technique/approach.