This is one of the most common questions asked by people, once they get through the "how do I control my buoyancy" phase of question-asking.
Attacking gas consumption by trying to change your breathing pattern consciously is not productive, and may even backfire. The problem with breathing is that you have to do as much of it as you have to do; trying to create a mismatch between your level of exertion and your breathing rate just results in bad things, discomfort and anxiety.
How much gas you need to run through your lungs in a given period of time is entirely set by the body's intense need to keep the arterial carbon dioxide level within a very tightly controlled range. CO2 is a product of metabolism, so the more you are metabolizing, the more CO2 you are making, and the more air you have to run through your lungs to get rid of it. So how can a diver control their metabolism? You can't turn the rheostat down, so all you can do is minimize the muscle you are using, so you aren't doing any more work than necessary.
The ideal diver can hang in the water for as long as he wants, until he wants to go somewhere or change his position. Then he makes a few efficient movements with his fins, creates the change he wants, and goes back to doing nothing. The new diver can almost never do this, because this degree of stability requires both control of posture, and a degree of adjustment of equipment. The posture involves keeping one's body long and flat from the shoulders to the knees, with the head up, and the knees bent to whatever degree is desired or comfortable, or balances the diver's weight. If a diver drops his head, he will tend to go head-down, and if he bends at the hips so that his knees drop beneath his body, he will tend to rotate head up (which is the usual thing seen with new divers). This is aggravated by gear setups that involve heavy weight belts, or lots of weight in integrated pockets set at waist level.
If the diver is in a position with head up and feet down, then what happens if he kicks? He heads up in the water column. Most of the time, that isn't desired -- the diver simply wants to continue forward along whatever structure he is observing. So how does he avoid moving steadily shallower? By keeping himself negative, so that the force causing him to sink is balancing the upward force being generated by his kick. Well, the net result here is that the diver is exerting a great deal of effort for no net displacement at all! He can't stop kicking, because he will sink, and he can't get neutral, because then he will rise. The answer is to fix the angle, so that any kicking he does propels him where he wants to go, and he can remain neutral as long as he wants to stay at the same depth.
The next step is to realize that sea life depends heavily on camouflage for survival, which means that moving rapidly underwater causes you to miss an awful lot of what is there. Moving slowly allows you to pick up on spots of color, changes of outline or small movements that betray the position of the critters you want to see. Moving slowly also reduces muscle activity and therefore gas consumption.
Only when you have become stable and begun to move with careful purpose, is it productive to address breathing pattern. At that point, you can work on slow and slightly deeper than usual respirations, more of the breathing pattern one might use during a meditation session. Too deep and too slow, and you will begin to see buoyancy swings during your breathing cycle. Too shallow and too rapid, and you are wasting most of the gas on the large airways (trachea and major bronchi) that don't participate in gas exchange.
Time spent balancing your equipment so that you can hover in a horizontal position is time well spent, and time spent developing a good, effective kick with no bicycling component to it is time well spent, too.
Awww I was so ready to launch into the physiology related to breathing... but TSandM's explanation is much more elegant than mine would have been
The only bit I can add goes along with the exercise suggestion - I've noticed that my air consumption becomes noticeably better when i'm doing some regular lap swimming. I'm not exactly an olympic level swimmer either, but the aerobic benefit and regular breathing patterns that you use when lap swimming seem to help! I suppose running would work too.... but I really hate running...
