To a certain extent the physical size of your lungs are a limiting factor, and there will always be 5`0" 98lbs women on their first dive ever who come up with more air than the instructor. In my experience: PROPER WEIGHTING and buoyency control make a big difference. Lose any extra weights and practice buoyancy. COMFORT level and relaxation underwater make the biggest difference, this comes with time and experience. BREATHING: there are many tips about breathing, some contradictory. For me the goal is to become so comfortable that breathing is normal and natural as it would be on the surface. ENERGY EXPENDITURE if you are cold get a better wetsuit, try not to thrash hands/bicycle feet or swim hard. There are other factors like physical fitness, smoking, stress level, illness etc that can affect air consumption but I guess when all is said and done, relax, enjoy your dive and don`t worry about it, some of us are just air hogs (I am) but it will get better.