The following simple breathing and visualizing exercise will help you relax and eliminate stress and negative feelings.
Imagine that all your negative energy, mistakes, misunderstandings, and emotions are black smoke that you expel from your body while you exhale completely.
While inhaling, imagine that all the positive energy of the universe is a pure, white light that you are taking in to every cell and atom of your body with the breath. Take a few moments to enjoy this clear, white light.
If you feel comfortable, use every breath to inhale light and exhale black smoke and negative energy.
If you are distracted by other thoughts, simply observe them without becoming emotionally involved while transforming them into black smoke and breathing them out.
Source: Buddhanet.net - Guided Meditations - Teaching Resource