As others have said, the key is to relax. Be relaxed beforehand. If you get excited during the dive and your pace of breathing increases try and slow down, relax, slow the breathing. Try not to over exert yourself in the water or get too cold. I have not done this and it might sound crazy but if you have a particular mellow tune you like, maybe sing that in your head. I know that kind of thing helps me in various situations, in and out of the water. The cardio comments is a good one. That will definitely help you have a slower heart beat, which may help with breathing and air consumption. (Note: I am NOT a Doctor.)