When doing breathing exercises you can imagine that you are breathing not only with the lungs through the nose and throat, but breathing with your whole body through the pores. Imagine the breath passing in and out of your whole body. The sensation will be unmistakable.
(Marilia meditating in a retreat on the island of Paros, Greece)
You can also isolate certain parts of your body, such as the hands, or solar plexus, and imagine breathing taking place through there. This can in fact be done through any part of the anatomy of the stomach, the liver, the eyes, and so on. Such exercises should be done with caution, certainly at first. They lead the way to the control of astral and etheric forces within the aura (which is the way to develop the etheric and astral body), and also to the practice of healing.
In ordinary breathing you can tie in this exercise before beginning to meditate or pray; as you breathe in imagine good forces coming in and as you breathe out visualize the unwanted things within yourself flowing forth. If you wish to imagine this in terms of colored light, do so; it is a practice leading towards the techniques of astral visualization. Of course you do not have to visualize the outgoing breath as carrying waste and unwanted products; in certain spiritual processes one becomes the mediator for spiritual forces and then the outgoing breath, which can be directed, is felt to be full of healing or other beneficial qualities.