Here is an excerpt from my book, The Silent Master: Awakening the Power Within:
You may find visualization difficult: Just as extraneous thoughts try to crowd out the focus of your concentration, many other pictures and images may intrude on your visualization. Without resisting or wrestling with the distraction, just gently return to your visualization every time you catch yourself wandering. Soon your mind will obediently learn you won’t accept intrusions. And of course, there’s nothing wrong with using both words and pictures in your visualization.
