Well, the very best thing to do, as the Lackadaisy tutorial says, is to *watch people*. You find body language and expressions in real life, and even movies, that you just aren't going to find in a tutorial. You can't give life to artwork by avoiding life.
Went through my favorites and found some good tutorials. Sorry for the long list of links. Some of them (eyes, noses, hands) are repeats but it is good to see how others depict the same things differently.