Optimizing the dive time for a given size of tank is mostly about efficiency.
Efficiency begins with an efficient breathing pattern. On land, where the gas supply is infinite, we don't have to worry about this at all, but underwater, things are different. The mouth and pharynx, and trachea and major bronchi, do not participate in gas exchange in the body at all, but we have to fill them and empty them with each breath. The volume used to do this is "wasted", but unavoidable. Therefore, the fewer breaths you need to take in a minute, the fewer times you "waste" that volume -- but since the amount of breath you need to keep your CO2 level normal is fixed, if you take fewer breaths, they have to be deeper ones. So one would think that taking the deepest breaths possible would be the ideal solution . . . but in fact, doing that, especially if you are a larger-framed person, results in a lot of instability in the water column, which can negate the benefit, because you're filling and emptying your BC all the time instead. There is a certain volume/rate relationship that optimizes gas usage efficiency, and we find that by experience.
Beyond that, reducing gas demand is about reducing CO2 production. CO2 production is related to activity level, so the harder you swim, the more you have to breathe. Since the force required to move against water resistance is proportional to the square of the velocity of the object, the work you are doing to swim goes up very fast as you move faster. (Note that streamlining your body and your gear will help with this, but only in direct proportion, rather than geometric.) When you are getting down to the fine-tuning stage, you really want to reduce ALL extraneous motion, and reduce drag as much as possible. Cave divers are sort of the ultimate expression of this, as we are always trying to figure out how to get more of a dive off a given volume of gas. Therefore, you will notice that cave divers streamline their equipment, dive in absolute horizontal trim, do not use their hands, and use a kicking technique that involves a long glide phase, where you are doing very little work. Adopting as many of these ideas as you can will help reduce your gas consumption.
With all that said, there are two important things left: One is that making artificial alterations in your breathing pattern to try to conserve gas can result in CO2 retention, and this is quite undesirable or even dangerous. CO2 is narcotic and makes you stupid, and in addition, in most people, it also increases anxiety and can predispose to panic. So the second thing is that there is a level below which you cannot reduce your gas consumption without suffering elevated CO2, and once you have reached that point, you have done everything you can. My favorite dive buddy is a 6 foot man without an ounce of fat on him, whose trim is textbook and whose imperturbability in the water is amazing. His SAC rate is about .7, and it is what it is. He turns the dives, and we both accept that.
