One thing I dont' see here (and I haven't read every single thread) is really key - relaxing. Eric F recommends long, motionless surface intervals with deep relaxation and natural breathing. While I'm not diving to near the depths he does - it's a huge help. A normal, comfortable, dive for me - say in the middle of a series with a marginal breathe-up because I wanna get a picture of something is 105-120 seconds. This seems to hold true anywhere between 6-15 meters depth with a pair of soft fins. I've just really started implemented Eric's protocol and haven't really fully done a proper breathe-up yet - air temps are very cold right now - and it seems to be windy most of the time when I'm out there (I've actually had my snorkel ice up!!) But I hope to add at least 30 seconds over the next six months or so without straining. Right now all I do on land is bagua and taiji - little pranayama here and there. I do generally dive at least once a week all winter - more in the summer.
Another important aspect is economy of motion. Once you tune into it you can feel how even a stray head movement costs you.
Another important aspect is economy of motion. Once you tune into it you can feel how even a stray head movement costs you.