This is a very common question from newer divers, because we ALL want to spend more time down there . . .
Most people who haven't delved into this think that how much air they breathe is determined by how much oxygen they need. That's actually not true at all; the urge to breathe is driven by the body's intense need to keep the level of CO2 in the blood very closely controlled. The reason for this is that CO2 plays a critical role in the acid-base balance of the blood, which has to remain within tight limits for all the chemical reactions of metabolism to proceed as they should. CO2 levels in the blood are directly proportional to the volume of air that goes through the gas-exchange portion of the lung, which is the small air sacs called alveoli. Air you inspire goes through the trachea and major bronchi, which are made of cartilage and don't do any gas exchange, and then into smaller and smaller airways until it reaches the alveoli.
If you think about it, this means that if you breathe in rapid, shallow pants, most of the air you are moving in and out is only encountering those cartilage passageways, and is not getting rid of any CO2. This is called "dead space", and if the breaths you take are small enough, you're only ventilating that. If only ten percent of what you breathe gets down to the alveoli, you have to breathe ten times as often to move the same amount of gas through the important part of the lung, and your tank won't last long. That's the genesis of the advice to breathe slowly and deeply -- it's trying to counter the anxiety-driven panting of new divers.
But once you have gotten through that initial "OMG, I'm gonna die!" reaction, new divers are still likely to have high gas consumption. Why? Because they are inefficient, and the more you move, the more CO2 you make. Your goal is to move through the water with as little effort as possible, in a relaxed state. That's what will minimize gas consumption.
So, how do you cut down on what you do? First off, work on staying in balance. Make sure your gear fits, so you aren't trying to cope with a tank that wants to do an end run around you. Don't swim with your hands, because they're very inefficient -- fins have huge surfaces to grab the water, and give you a much better return on your effort investment. Don't swim fast -- the more you zoom around, the faster your gas will go. And fix the number one cause of inefficiency, which is improper trim. If your body is aimed upward and you kick, you will go up -- if you don't WANT to go up, the only way to avoid it is to be negative. So you then expend a lot of energy kicking upward and getting nowhere. If you are horizontal, any kicking you do drives you FORWARD, which is where you want to go.
You don't have to be thin to have good gas consumption, and you don't have to be strong or even particularly fit, although those are all laudable goals. You have to be relaxed and efficient. It does come with time, but if you don't fix things like improper trim, all the diving in the world won't reduce your gas consumption beyond a certain point.