When I started diving I had terrible air consumption, mainly because my instructor over-weighted me and I was swimming way too fast. Fast forward to now, I became an instructor and my air consumption is great. I improved by experimenting and diving a lot, but in the end it comes down to being calm underwater, both mentally and physically.
I swim super-slowly (some customers even complain when I am guiding them, usually the same ones who miss all the marine life, but most people end up seeing a lot more). My advice is to do everything in slow motion. Only move if you really have to. When you do kick, move your legs slowly. Put your arms in a comfortable position and don't move them. I keep my arms crossed over my chest or hold my hands together in front of me under my chin, this also helps to life your legs up and give you a better horizontal swimming position, reducing drag. The dive should basically be a constant hover, too many people swim to maintain their depth in the water because they are not neutral. Make sure you are weighted correctly, many people can do with taking off a couple of lbs/kgs, usually helps with air consumption. Make sure your gear is streamlined and that you are horizontal. What type of fins do you have? I would recommend rigid fins with a strong blade, not fins that bend easily and not split fins. Personally I use Mares Avanti Quattros, they work very well if you ever do have to kick against a current and you get a lot of propulsion from a single kick. There is a reason you see a lot of instructors with hard fins.
A final piece of advice from me would be to consult a free-diving instructor. I know a lot of people who have improved their air consumption dramatically just by going through some basic breathing exercises. The free-diving instructors tend to have really good advice, as they make their living from making every breath last!