Good method/approach in improving advanced Drupal development skills
I have found the Drupal Examples module to be a great starting point for learning enough to develop my own modules, or to aid in solving a particularly thorny problem I'm having.

There's also the Devel and Dev Themer modules, which help to illuminate some of what's going on behind the scenes.

Then there is the API documentation, which I personally have a harder time understanding until I read some of the comments.

These are the ones I use most; others will probably chime in with others, which I know about but lack enough experience in to form a helpful opinion.