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

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).