Breathe out during the "effort"/exertion phase
Exhale on the exertion (or most difficult part of the exercise) and inhale on the recovery (easiest part of the exercise). For example, when doing a crunch, you should exhale when you lift your shoulders off the ground, and inhale when you lower your shoulders to the ground. When squatting, exhale when you stand up, and inhale when you squat down.
Instructors will often encourage their students to think “inhaling on the preparation for the move,” and “exhaling on the execution of the move.”
Do not hold your breath during weight work
Holding your breath while lifting a heavy weight can push blood pressure up significantly. You can also reduce blood flow to your brain and increase pressure in your chest – both of which could be dangerous while exercising. This could cause a heart attack, a hernia or a stroke, not to mention dizziness and fainting. Always breathe!
Exhale through your mouth and inhale through your nose
This technique helps to control the pressure of the inhalation. To prevent gulping air, using nose inhalations will regulate the intake, which in turn, regulates the oxygen levels in your blood. When you exhale, it is perfectly fine to "puff" if you are contracting your muscles quickly.
Concentration and Breathing
If you are new to lifting, start out slow and practice proper form and breathing techniques with light weights, until you are sure that they have been mastered.
When correct breathing becomes your second nature, it's a good idea to concentrate on the primary muscle or muscle group you are using while lifting the weight. Concentrating too much on breathing will reduce the effectiveness of the exercise.
Even when correct breathing becomes your second nature, step back and analyze your breathing techniques for each new exercise to avoid forming bad habits.