I often stress the importance of taking a big breath before you play, like a singer. I need to specify that deliberately taking in air, forcing air into your body is almost as bad as not doing anything at all. Don't think about taking the air in. Instead, empty your lungs and then just relax your whole upper body and let the air come rushing in by itself.
Most importantly, make sure that you are breathing with your diaphragm and not with your shoulders or your ribcage. Your lower abdomen should expand with each intake of air, while your shoulders and chest remain in a relatively calm position.