I have in the past written about how a good way for a person to understand the intricacies of the human face is to practice "copying" photos that they enjoy. I don't mean tracing them, I mean looking at the picture and attempting to recreate it on a sheet of paper next to it, at the beginning stages it can even be beneficial to use a program that will place a grid on the face which can also be placed on the sheet of paper, so that someone can tackle each part of the face piece by piece. This is a great way to gain ability of the various curves, shadows, and wrinkles that appear on all faces. After a while it becomes like second nature and you begin to understand how most people have similar lines on their faces, regardless of where they are from or if they are male or female.
Drawing a live model can be a lot more difficult because even the most stable person has a hard time maintaining the same expression for the amount of time that it takes to recreate their image on paper, so when attempting this, it is important that you already understand how the markings of a person's face are without needing them to hold the exact pose. A person is not a photograph and try as they may to replicate the same pose over and over, there are going to be slight differences that end up appearing during each pose.
While I have been doing this long enough to teach others in the process, I am by no means world-class. I have my shortcomings as well and this is something that I felt when recently trying to replicate a person who was working at a coffee shop.
Before anyone thinks I was being creepy it was the waitress herself that noticed me working on something else and actually asked me to draw her, not the other way around.
The end result is something that I was kind of pleased with but I will admit that I was being kind to her and intentionally not putting any wrinkles on her at all since she was in her 20's. She in fact did have some wrinkles but I intentionally skipped them. She was kind enough to hold this position for me a couple of times and once again, I was not being creepy, she chose the pose, not me.
I also had to finish this in a reasonable amount of time so it could be considered a "rush job."
In the end I gave her the picture and she was really happy to have it. That made my day and I hope it kind of made hers as well.
This is something that I think people overlook about art. It can bring joy to other people as well as the artist. I suppose this is why street caricaturists are so popular in tourist areas around the world. Speaking of which, that is a skill I never developed and hope to be able to work on some day.