To watch without a picture.

No,it's not ancient Chinese philosophy. This idea was first suggested to me by my prof in audio production class. You simply put on a movie (or a film depending on your personal tastes) with something draped over the TV set (or, for those of you with a bitchin-ass home entertainment setup, turn off the television) and listen. High-quality headphones are a plus (especially for those of us with ADD) since they provide a higher level of immersion.

Even if you've seen it a hundred times, you'll probably notice new things about the audio track.

The idea here is, obviously, is to move your focus from the most dominant of the five senses (or six, depending on your spiritual beliefs) to the runner-up. Not only does it broaden your aural observation, but also, if done enough, teaches you to focus more on sound in everyday life, an essential for anyone involved with an audio profession, and a nice perk for anyone looking to improve their powers of observation.

One of the first things I noticed through this activity was how often sounds from certain sound libraries are used in various pieces of media. If you play any video games, you've probably already discovered this. When playing a game, you're assaulted by the same sounds repeatedly, until they're so beaten into your subconscious that one instant of recorded audio has become a part of your soul. Then, you hear the sound on a TV show, or, in a movie that used the same sound library and you notice a sound because it's so familiar to you. (This is a pretty jarring experience, especially the first time, which might prompt you to spew epithets or gesture frantically in the general direction of the TV set from whence the sound came.)

Among the sounds that seem to be most popular are the mechanical UFO doors used in X-COM and various gun-handling noises from Duke Nukem.
After a while of listening you'll be able to discern between different microphone types, recognize subtle changes in enivronment, and even notice mistakes or shoddy work, among a great many other things that influenced you before on a subconscious level.