Breathing in Asana
Our natural tendency is to hold the breath or use stress induced breathing while holding a posture. This creates stress in the body. To create relaxation, breathe slowly and deeply in all of the asanas. There are several ways to breathe in asanas, and these will differ with teacher and tradition. I like to emphasize different breathing techniques or pranayamas while holding different postures.
Use Dirga pranayama in most postures. Focus on actively breathing into the chest in backbends and chest opening poses (pigeon, yoga mudra, warrior I, fish, bridge). For forward folding postures and belly down postures you can focus on just breathing into the belly (child, forward fold, cobra, boat)
Use Ujjiaj pranayama when holding strength building postures to increase endurance and focus (downward dog, standing squat, warrior III, warrior I, sun salutations).
Use Kapalabhati Pranayama when you want to intensify holding a pose and to strongly activate the prana in the body. Kapalabhati can also create a strong focus during an challenging pose to allow us to stay present with the posture.