I see this question often, especially in the context of the front-end. From my experience, the best way to improve your coding skills is to create something that one finds personally interesting, as that maintains your motivation. Open-sourcing it and posting to sites like HN is the best way to get honest, open feedback.

Do you think it is counter-productive to follow the advice above? Should we all be submitting PR's for bug fixes on large open source projects? Or should we only be allowed to work on projects that have been vetted and added to some dev community's backlog? I'm serious because I see this question come up so often.