I suppose that most was said already. Anyway:
*Reduce excessive weight. Don't need it don't take it. Increase weight=increase air consumption. It is a common mistake to think that the BC will compensate for any weight that one takes. It will balance you vertically, but in order to move all your mass forward you'll have to use the force of your legs. It's just simple phisics (newton's law) that we have to keep in mind.
Excessive weight will also increase your bouncing even when balanced. You inhale air you rise , you exhale air you drop down. Once balanced, you are supposed to use your breathing to make small corrections. With too much weight you'll drop like a stone each time you exhale.
I think that excessive weight is the #1 "dive time shortener" for most divers.
*Try to be "slim". Any dangling pieces of gear only increase drag (and also increase tangling potential and/or raise sand, break corals).
*Move slowly. As less as possible, like in slow motion. Remember that water friction/resistance increases with speed! Increase your speed=increase resistanse=increase air consumption.
Don't move your hands at all, if not needed.
I think that moving too much (especially hands) is #2 dive shortener.
*Relax. You'll enjoy you dive better and have more time to enjoy from it. Whenever you see that you are running too fast, breathing too fast stop and calm yourself. It will also help you if any problem arises, and let you stay far away from the diver's enemy, the panic.
Myself, keeping the breathing pattern described here so many times helps me to relax and have more time to look at the surroundings.
Increasing you tank volume? I don't know...It's heavier, bulkier. Perhaps you don't gain as much as one would expect. Better trying other alternatives before going on the heavy tanks.