Listening is like any other skill: Reading about it helps, but practicing it is everything.
Another well-regarded technique is sitting somewhere and writing down every sound you hear. I take that a step further create a description for each sound, both on a technical and emotional level. This can be done in a wooded forest glen, a concert, a coffee shop, the bus...anywhere. Personally, I keep a journal with me at all times to practice this skill (a smartphone with a note-taking app is good, too). On the technical side, I make guesses as to the frequency ranges of certain sounds, and check them against a sonogram or spectrograph when I get home (assuming I'm recording what I'm listening to). On the emotional side, it's just a personal heuristics thing, but forcing yourself to assign emotive descriptors to everything you hear can be challenging...and then figuring out why certain sounds make you feel a certain way is another introspective challenge unto itself.
A recent workshop also reinforced a zen-like koan that I think is essential for all creative endeavors: Don't be disappointed if what you're hearing isn't what you're after. Bend your mind around the circumstances and embrace all sound. If you do that, everything is music and nothing is merely noise, and no recording outing will be disappointing (or at least every recording session will teach you something).