Read the source code of stuff you are already using. Dig into that networking library, or explore that templating plugin.

This not only lets you read more code, but also lets you pick up knowledge in areas that are often taken for granted (no need to think about X, there's a library / plugin /framework for that).

I want to second what div already said, with the added comment that it's better to have a specific goal than to wander aimlessly through the source.

Every piece of software has bugs and limitations. When you encounter one, go figure out why it is the way it is, and maybe even fix it.

If all your dependencies are so perfect that this never comes up (hah!), you can always pick a project you use and like and look at their open issues. Begin trying to solve one of them.

Thank you so much for your reply! I like your advice, I think it's a great place to start.

Learn new paradigms like functional programming (the purest, the better). It'll teach you how to "bend" your mind around certain problems/structures.

Thanks, I agree. Learning Clojure has certainly been a mind-bending experience for me so far!