Sit somewhere comfortable, either in a chair with a straight back, feet flat on the ground and in an equal position and hands relaxed on the arms of the chair, or stand in a straight position with your legs slightly apart. Inhale slowly and deeply through the nose and hold your breath for a while.
Imagine that you have a straw in your mouth, give a strong and sharp short exhalation, without forcing all the air out of you. Repeat until all gone. With each short exhalation, visualize the negative energy as little black bugs flying out of the "reed" and falling dead to the floor. With each inhalation, visualize purifying light energy entering your entire body. Repeat 8-10 times.
This technique can also be used to strengthen the lungs, to revitalize the body, but also as part of the ritual cleansing before any Mystical work.