Do not breathe out all in one go. This will cause your lungs to require oxygen quickly and you consume more than you expect.
This doesn't make any physiologic sense.
The amount of air you HAVE to move through your lungs in a minute is determined by your production of carbon dioxide, and that's determined by your basal metabolic rate, combined with the amount of muscle effort you are using. (That's why slowing down helps your gas consumption, because the less you move, the less CO2 you make.) But this is the amount of gas that has to go through the small air cells in your lungs, so the air has to get down there to be effective.
The air moving through the upper airways -- your larynx, trachea and major bronchi -- does not participate in gas exchange, and is termed "dead space". If you breathe very shallowly, a large percentage of each breath simply ventilates the dead space, and only a small fraction gets down to the air cells. If you take a slow, deep breath, a much larger percentage of it is getting to where it needs to be.
Watching students in the pool this week, it was so clear to me how many of them are just "huffing" little tiny breaths in and out very rapidly. This is extremely inefficient ventilation. ANY technique which causes you to slow your breathing and take longer, deeper breaths, will help your gas consumption. I think yoga is an excellent suggestion, because it teaches that kind of breathing pattern.
In addition, breathing that way actually helps you achieve relaxation. There is something very meditative to slow, deep, rhythmic breaths, which is undoubtedly why they are part of yoga and meditation techniques. Achieving relaxation underwater also leads to reduced movement and less CO2 production, so this is a very constructive spiral to get into.
Good buoyancy, horizontal trim, efficient propulsion techniques, and slowing your speed through the water will all have enormous impact on your consumption, but a reasonable breathing pattern is a requirement, as well.