This is an excellent discussion so far, and I'll just jump in here.
I use Chi/Ki and Pranayama(lit. -Breath-Mastery, Sanskrit) models while meditating and for martial arts. I also gave a short talk about the breath techniques I use @ the last KallistiCon.
So, first I'll rant about Breath (and I'd e-prime this, but it's freakin' late, and I'm starting to feel the sleep dep, so keep in mind that I'm talking MODELS not Truth). I'll go further into this and how it ties to Chi/Ki in further posts. Since most people here should be somewhat familiar with the 8Circuit model, I'll stick to that for most of my explanations. If anything confuses, please ask.
-Breath: All Chi/Ki building exercises I've done and read about start with a breathing practice.
Why? Breath is the door to achieving conscious control over sub-conscious bodily processes.
The body has many systems and processes that we usually let the subconscious and the limbic system handle. Heart rate/circulation. Blinking. Digestion. Breathing.
Yet breathing is the one that we can have the most direct conscious effect on. We learn very early on (bathtub anyone?) to hold our breath when necessary. Some websites I've looked at says a human can learn to hold it's breath at 6 months of age. It's also one of the primary functions that we first start doing _outside_ the womb.
In the 8Circuit model, this is imprinted 1st circuit reflexes. But, it's also the one of the easier imprinted reflexes to achieve conscious control over. Once you can mindfully sit and follow your breath, you can mindfully sit and follow more complex bodily reflexes, like heart-rate/circulation, and thinking. Again, breath is the 'subconscious' process that we can most readily access consciously through our motor cortex. Yet, like training the hand to draw, training the breath takes some direct, repetitive practice.
Now, once one achieves a relaxed and mindful awareness of one's breath, the next thing you should notice is Muscle Armor (1st circuit tensions). These, according to the 8C model, are chronic stress-handling "muscle-armor" tensions. In "civilized" hominids, these usually occur in the upper frame and shoulders, as well as the abs and lower back.
Vividly imagine someone yelling and swinging a baseball bat at your face and these areas of muscle tension should leap into your awareness.
With these areas tensed, the body breathes by raising and lowering the shoulders. This breath pattern tend to increase noradrenaline and the related responses in the limbic system. This is natural for a monkey to do, but the constant perceived threats we encounter in our "civilized" environments, which we then mentally suppress, have conditioned these areas to be chronically tensed. This leads to a constant flush of noradrenaline in our systems, which may lead to some nasty results later in life.
This is not the first way we learn to breathe, and closely observing babies will tell you this. This has been trained into us from constant exposure to perceived threats. So, most breathing exercises teach you at first how to recapture the breathing techniques of the innocent child.
Wow, that got kinda long, so I'll go into the practice and effects of the "Complete Breath" tomorrow.