Most people Do Not Love this answer, but drawing from life, and anatomical study, mostly.   All my life drawing classes were really valuable, and I still consider the two-semester anatomy class I took studying muscles and bones to be one of the best ones I’ve ever taken.  When you get immersed in that enough, you internalize several sets of proportions, which then makes it easier to tune into people’s unique configurations as well. 

There’s a basic ‘in general’ kind of proportion template for a person, and once you know it, then you can start looking at the ways people adhere and deviate from it.  I think that was pretty critical to any skill I may possess in the likeness department.  Sometimes it’s the shape of the features, and sometimes it’s where they’re located in relation to everything else and without a base shape to project from, you’re kind of flailing in the dark and hoping to hit. 

Life drawing is also good for discovering where you’re drawing symbols for things versus drawing what you can really see.    This is particularly a big thing for eyes if you’re trying to do something more real.  

So, basically, learn about internal structure, and give yourself the opportunity to draw what you really see, not just what you think you see.