I really agree with the output aspect. What I find really works for me is getting stuck into 'projects'. In the recent case its been comics. My goal for November (NaNoWriMo but comics) is 100 pages in the month but I'm dividing it up into issues. I've improved a lot. Also its helped a lot of my other skills you don't really practice when doing general art. Like writing, pacing, shot composition etc. It's part of a general thing I've been trying I've called 'modeling'. That is I select someone who does something I wish I could do. Then break it down into a set of behaviours. And for professional artists, what has become clear to me is you need to put in the hours. But be careful, still find time for play and fun drawing or you'll burn out.