I totally agree with Perpet1; in my experience, there are two things that will help new divers extend the amount of bottom time they can get on a tank of air, and probably the most important is comfort level. If you are relaxed and comfortable in the water, your air consumption will go way down. I've heard divers telling students how to control their breathing before, but from what I've seen, the best results come when you no longer have to THINK about breathing, when it has become natural. Then, instead of gasping in short, quick breaths, the inhalations become longer, and exhalations become a slow trickle of bubbles.
The other thing you can work on is increasing your general fitness level. There's a whole different type of stamina required to do multiple dives on SCUBA. Even otherwise fit people will find diving exhausting at first (although I'm sure part of that is mental, as well). If you can increase your stamina, you will also decrease the amount of air you use.
Both of these will come naturally with experience to some extent, but you can also consciously work to improve them.