I recently went through my whole collection of old sketches from the last four years, and noticed that I've deviated somewhat from my original goal of learning to draw good stuff without reference. Over the last two years I've increasingly drawn from photos, and it's been a long time since I really tried to do anything "free". As a result, I tend to still struggle with those things.
To remedy this, I've decided to do more of it. Starting with this page of 50 faces drawn without reference. I scribbled 50 circles over the paper and then drew an improvised face/head on each one. Most of them took 2-3 minutes.
The process and results were actually better than I had feared. It was kind of fun, and not too difficult. My plan is to do another page of 50 faces from reference grabbed out of a video clip, featuring the same person from different angles. After that I'll try another page of 50 from my head, attempting to get some of that person's character/look into the sketches.
As a fun comparison, here are a few similar head sketches that I found from almost exactly four years ago. While I still haven't mastered the human head, I think it's safe to say that I've at least improved considerably.