How do you train your ears?

How do you educate yourself to refine your ability to listening to music and being able to tell about the details of the sonic nature?
I guess, first off, one has to listen to lots of music on lots of different systems, and catch intrinsic details and subtle differences. Knowing basic music theory and being proficient in one or more musical instruments would also help.
However, simple listening may not improve one's ability unless the listening practice is guided by educated practices that have been exercised by experts and those with golden ears.

How have you refined your hearing/listening capability?
Any good source you know of to recommend to novices and enthusiasts?