Adam, I did that using natural abdominal breathing. Although reverse builds qi more quickly, natural has always seemed more calm to me. I've learned to calm my reverse breathing down substantially since then, but still not as much as natural. My mind's focus is usually just on the physical feelings and sensations of executing the perfect breath for your given state (e.g. you need to work your way to longer breaths and the mechanics seem to differ very subtly from a shorter to a longer.) Really, once you hit ~40 seconds it doesnt even feel like you are breathing any longer - its more just calm respiration, all movement is abdominal and everything above the diaphragm is totally relaxed. Lungs, bronchi, trachea, sinuses - when they are all completely relaxed and not 'adding anything' to your breath, then turbulence in your breath all but disappears.
Vortex, Embryonic Breathing is the foundational qi building breathing practice that Yang Jwing Ming teaches. He's the first author I was exposed to on the subject of qigong, bout 4 years ago. I picked up The Secret Of Youth and after reading halfway through it I discovered why it said 'an advanced regimen for the serious practitioner' on the front

I thought I'd be able to hop skip and jump through shit like I have always done with stuff. I started from the beginning after that one! I read about longevity breathing in BKF's Opening the Energy Gates of the Body and applied the concept to Yang's EB, quite happy with the results of those two simple things.