Take a general Drupal content management course or have a custom course developed for your team, providing a lasting resource for future employees.
This is our full course catalog. Introductory courses require no prior knowledge of Drupal. They may have other prerequisites of prior knowledge or experience. Intermediate courses require Drupal in a Day or similar level of experience. Advanced courses require previous experience with Drupal site building.
I thought this course was extremely enlightening. I think I achieved my goal of becoming more competent in looking at/modifying/creating modules. Trainer was ridiculously qualified, expert, patient, and informative.