I read a tip that was helpfull to me. Before you descend, put your snorkel in your mouth and spend some time on the surface with your face in the water, just looking at (or towards) the bottom, breath through your snorkel and try to relax. do this for a few minutes, perhaps on the way to the anchor line. Then when you're ready put your reg in your mouth and descend. This technique relaxes you befor your descent and relaxation is the key to low air consumption.