Not strictly related to illustration, but with music, I just practiced. I tried new things and I tried to learn new things and get good at those new things I learned. every so often I would take on a bit bigger of a project to test myself and see what I need to work on.
ShangXian
In my case improvement comes from practicing what I am trying to learn and master, being inspired by other peoples' works, listening to feedback and critiques, using tutorials, looking at references, trying to experiment by myself, trying to come out my comfort zone and having fun.