So there are probably a lot of people out there that want to know what you can do in order to make yourself a better Drupal developer. We if you are like me then the first step is to start yourself a huge Drupal project that should have at least 3 or 4 developers but only has 2. Then, one day have one of of those two developers come into work to let you know that they are leaving for a Drupal Job in Manhattan working for Sony. :) (you know who you are). I'll tell you what, you learn to find as many resources as you possibly can in order to breathe, eat and sleep Drupal.
So there are a few references that have helped me get to where I am today in my quest to become a Drupal Ninja. The first would have to be the Pro Drupal Development book by John K VanDyk and Matt Westgate. This book has not left my side for the last 3 months. I have read it a couple of times and reference it quite often. My book is even signed by Mr Matt Westgate himself. (Jealous?) He signed my book while in Chicago attending a Lullabot conference.
This brings me to my next place to gain a wealth of Drupal knowledge to be a proficient Drupal Developer, a Lullabot Workshop. These guys and gals put a fun twist on learning about Drupal. Go to a Lullabot Workshop, you won't regret it! Plus, if you go and give them some feedback, you will get a great Lullabot teeshirt. Hrmm... I never did get mine!
If you aren't able to make it to a Lullabot workshop in person than I would highly recommend the lullabot podcasts. These light, fun and educational podcast will really boost your knowledge of Drupal contributed modules, core Drupal, people involved in Drupal and give you an overall better Drupal vocabulary. (Not just taxonomy vocabulary!) You don't have to start at podcast number one, but I did and I lived to tell about it!
The Drupal.org forums. Go out there an Google your question on the forum. In Google try the old site:drupal.org/forum question and see what kind of results you can get. Also, if you learn something and find someone having a question, feel free to answer a question yourself.
Another feature of the drupal.org website are the Drupal handbooks. These handbook pages are created and maintained by some of the registered users of drupal.org. I have even had a little time to sit down and edit a few pages and have even contributed a page called Customising the search results page. I hope to be able to contribute more in the near future in order to help to increase my Drupal karma.
The Drupal Dojo is a good place for watching video cast of education pieces. I have only watched a select few of these so I don't have a lot to say. I have a feeling these are a must watch in order to be a Drupal Developer and wish I had some more time to do so. I will be putting this on my to do list for sure.
IRC is a great place to ask questions, view others in discussion, and get a good overall feel for the Drupal community. There are a few IRC channels to choose from and try not to ask the wrong questions in the wrong channel. Most will welcome you with open arms but there are a select few that will let you know if you are asking an inappropriate question. The Drupal related channels on the freenode network are #drupal-support, #drupal-themes, #drupal-ecommerce, #drupal-dojo, #drupal-consultants and #drupal-dojo.
There are many other reference materials and sites out there but the ones I mentioned here are the ones that I have used the most, at least to date. With the popularity, and Drupal developer base growing these references are always getting better. And how they get better are by the ones that use them the most. Contributing is the key to a successful and great open source program like Drupal.