I've found animation character sheets in particular to be helpful. They contain lots of different face shapes from every angle, making every expression, in simple line art!
I've gone to cafes and tried cartooning a few people around me while I drink my coffee. First I draw them fairly realistically, then I work off of my own drawing to push their features further and further and imagine how they would look making different expressions and from different angles.
I was in a slump on Monday, so I tried that Kreugan tutorial linked above and traced photos of male faces in Photoshop (pulled from Facebook to get some real faces and diversity, haha....not sharing!). Then I made the background layer invisible and studied my own line art, looking for the distinguishing features and proportions between different people's faces. From there I tried cartooning them by hand in a sketchbook to see if I could retain what made them look like them. It really helped me to pick up little details I can employ, like that 'v' or 'u' shape some people have in the center of their upper lip.
I put my sketchbook and my favorite tools next to my bed and I've filled a page every night before bed for a couple of weeks. Here are some colored pieces:
I'll keep working on it.