Something I don't think I've seen said too much is that I think the limiting factor for art improvement by far is "vision"/observation rather than any concrete skill like line quality or similar. You can't "fix" what doesn't seem wrong to you, after all. It can feel like something is off, but you can't put a finger to it, whereas with stronger vision you would be able to identify specific things that need to change. That can be as simple as proportional adjustments like placement of the eyes on the skull or something a little bit trickier like realizing the image is general unbalanced in values and the overall "exposure" needs to be brought up or down. Honestly I think this is the problem a good 80% of the time for artists but I guess it's not brought up as much since it's pretty ambiguous as to how that ability is improved aside from saying something like, "observe more," which is not particularly helpful. Regardless, I feel like "improvement" in the arts in general is pretty different from other skills because of this. For other things the general goal can be quite obvious, but for art you have to figure out what you're even trying to accomplish first (you can't "fix the eye" if you don't know it's not in the right place). Aside from that I felt like half the time I improved the most just by taking breaks of a month or so (not really intentionally) and then coming back. Even pretty specific technique things like having nicer simplified shapes and values and whatnot improved a fair bit from times like these. In my opinion the reason for this is because every aspect of art you need to be a quite strong artist is conceptually very simple but putting them all together requires a more and more intuitive and holistic model, so refocusing with breaks can help with that quite a bit. Naturally good old-fashioned sharpening of your bank of patterns by drawing a lot of hands or whatever else from reference is important too but I don't think anyone has to be told that.