You've got an excellent grasp of facial structure in general, these sketches are very good! I love (and, as an aspiring comics artist, very much envy) your expressive linework.
One of the 30-second drawing techniques that's stuck with me since my college days is to pay attention to the shadows first. Block in where the mid-range shadows are. Then go back and fill in darker shadows. Then fill in details and fine-tune highlights, etc. The idea behind this is that shadows make up the majority of the form of the object, so focusing on those helps prevent getting distracted by details. Your mileage may vary, of course - I struggled to wrap my head around it, but I was not very disciplined with my practice in those days. :)
Also, if you want some in-depth tutorials on facial anatomy, the How To Draw Comics website has some really good free resources. Eyes, lips, noses, and even ears are broken down by muscle group, with explanations of how those muscles interact and tips for visualizing the feature in 3D. The art style it's geared toward is more realistic than cartoony, as well, so it would probably be a helpful resource even if your end-goal is more realistic portraits than comic art.
Keep up the good work!
3
1
