I was flattered to have been asked by Siobhan to contribute an tip for her article – this tip is something I believe is important when starting to work with any platform. You always use a system more effectively when you understand how it works.
Use the Codex to get you pointed in the right direction, but always go to the actual core code as well. There are many good reasons for this, but the most important is that the documentation says what the code is supposed to do, while the code says what it actually does.
Documentation can also lag behind the actual code (especially when core patches are merged in).
Also, reading the code is a serendipitous opportunity. While looking at one thing, you may see several others that are helpful immediately, and some that might stick in your memory for future use.