I also struggle with this. I think the trouble comes from having a vague sense that "I need to improve" and it is solved by saying "I need to improve at doing THIS."
Here are some things I have found that help. I'm writing this down as much for myself as for others.
1. Define a project
Yes, we do a lot of sketching and playing in artistic spaces, and we do learn from that. But to add structure to this, I find it best to start with a project in mind. The type of work is not as important, it is only important that you define it. You might not define it all at once, and instead make gradual steps towards that definition.
2. For each thing that you define about your project, you now have a set of things that you will need to learn. Gather reference or other information you need to learn about those specific things.
3. Apply what you learn to your project, making many iterations, until the part you have learned is sufficient to tell the "story" that you want your project to tell. That is how you will know that you have learned "enough".
4. Move to the next part of your project that you have defined and restart this process. Over many iterations, you will have set goals and learned what it took to achieve them, until the end result is the project. When the project tells the entire story you wanted to tell, then you can know you are finished and move to the next thing.
This is a practice I developed while working in 3D software, building each piece of a scene one by one, and I have carried it over to illustration and it seems to be working.
This process gives you something to focus on, and that focus is rewarded by eventual completion of each step. Additionally, and perhaps most importantly, by setting these obstacles and overcoming them, you will learn the way your mind artistically handles each obstacle and how you apply the things you have learned. All of the reference and learning you have in your mind will then be categorized by the experience of completing that part of the project. You will naturally cull the things that aren't working because it won't improve the story of your project.
Your project may start as a vague sketch, or it might be a fully formed idea in your head already, but the important thing is to define at least one thing about it and work towards portraying that thing. When the message is clear, ask yourself what is next in your project. When your project no longer needs defining, you are done!
If you want to test your project, I find that sending the results to a friend and instead of saying "Hey, look at this!" (to which they will respond 'Cool! Nice!' no matter what lol) ask them "Can you tell me what you see here?". You may be surprised that the things you were worried about don't even register to them, or that things you thought were perfectly clear are not. That just tells you where you've learned enough (for now) and where you should refocus.
It can be easy to get stuck on a certain part of the project. Maybe endlessly drawing a hand over and over, or not being able to get a face right. When this happens, I get it to the best that I can with what I know and just put a marker next to that to return to it later. Sometimes, the reason it's not working is not the reason you think, and it may be that you need to better define another part of your project before your struggle point makes sense. Other times, your brain just needs a rest.
I'm off to take my own advice now. Good luck!
It's ultimately up to the user whether they hold to the integrity of practice time, since I could potentially open a timed module and just let it run. I would only be cheating myself by doing this, and what would be the point?
A toggle or option on the viewer would allow someone to turn this off or on, at preference.
Studying anatomy will never hurt your understanding of the human body.
I am not sure why this question seems to pop up here, but perhaps somewhere it has been said that learning anatomy too early can be a distraction/hinderance? Knowledge of the anatomy of the human body (or of any thing you want to represent in art) will only hurt your understanding if you view it as a replacement for seeing.
I did make this mistake for many years, believing that because I studied anatomy I would not have to ever draw from life or reference. I was in my early 20s before I realized that this was a mistake. However, once I began drawing from life, I found that my knowledge of anatomy made it much easier to understand the forms of the human body as they tilted toward and away from the eye.
One also cannot underestimate an understanding of perspective. Especially when drawing from photo reference, since photos inherently contain a "second hand perspective". In other words, you must know where your eye is seeing from. Otherwise it's all just beans and bumps. I find that incorporating a simply horizon line into my drawings is enough to sort out the contours. And as others suggest, learning to draw contours will aid you greatly.
Anatomy and perspective should work together to aid you in seeing. Whether it's seeing something in front of you, in a photo, or in your mind...As long as you don't replace seeing with knowing, you'll keep growing.
As JCMLfineart says, it's a workout. You do a lot of working out. And then you go lift a car, and it looks effortless and easy. I recommend Iterative Drawing as a practice. So, taking one of the poses that you have done and do it many, many times. Not completing the drawing. Simply starting from scratch anywhere from 5-10 times. You'll notice that your understanding of the pose starts to change. You'll notice that you want to rely less and less on the reference. Keep using the reference, but ask yourself "What is it that I'm understanding now that I didn't before? What am I remembering about this pose? Where am I getting stuck each time? How am I getting unstuck?"
Good luck! Draw something!
Lot's of great advice above this. Especially in regards to getting back to fundamentals.
I find that the most important part of drawing is the voice that I use to talk to myself while drawing. Therefore, as others suggest, warming up with some fast gesture drawing is a great way to get started because...
a) you don't have time to think anything other than constructive questions like "is the elbow overlapping the knee? is the head horizontally aligned with the hand?" and then answer these questions with your drawing and...
b) it's not a finished drawing, it's just practice. Baseball players don't get to "keep" the homerun they got during practice. It's all building up to the big game and getting your brain aligned with your body to make the play when it's needed. And the play is all about how you speak to yourself while drawing.
Drawing is as much about seeing, whether in reference or in your own mind, as it is about the act of drawing itself. Thoughts like "I'm bad at drawing. Everyone else is better than me at drawing. This drawing is the worst drawing I've ever done. Am I suddenly bad at drawing for no reason?" are like walls in front of your eyes. They stop the process completely. The statements are subjective and yet absolute, the questions are not objectively answerable. Ask yourself questions that are related to what you are wishing to translate into art. Ask yourself questions that are objectively answerable. Answer those questions with the drawing.
Another factor that frequently occurs as you learn to draw better is that your idea of "better" changes. As your skill increases you hardly notice that the bar of quality increases with it. There is usually only a brief window where you are drawing objectively better and also subjectively better. These two are always in a race. You can get frustrated when you hit the point where your taste gets ahead of your skill, or you can rejoice in the fact that you now have somewhere new to get to. As Snump said, when you're here "you're already halfway there!"
And on that note, when you practice drawing try to have a clear goal in mind. I recommend looking into Sycra Yasin's videos regarding his ideas on Iterative Drawing. Decide something you want to work on, for instance today I am working on simplifying poses. I find some pose I like and then I just draw it over and over from scratch. I'm not trying to create a finished drawing. I'm trying to practice the movement, and in so doing I find that around the 5th repitition I gain a new understanding of the pose because I see where I am slowing down and getting stuck over and over.
Good work!
