Guard of the heart is a practice that is most helpful once we've made a resolution about something. When we want strongly to follow through with our resolution we should guard our heart from doubt and from counter experiences that move us away from our resolve. The fruit of guard of the heart is a heart full of strength and commitment to our vocation, our work, and our relationships. Since the heart provides us our innermost experience of 'being with' God, we should guard all our choices to 'be with' God.
An example of guarding our heart is to notice when we feel anger rising. Is there a pattern? Is it when we're with certain company? Does it follow after watching hours of football and drinking beer? Is it after we've watched certain TV programs? Once we find a pattern, when we're trying to decide how to invest our time and energies, we then can guard our hearts from those factors harmful to our calm mind and our prayer. When thoughts rise that incline us to anger, then we practice guard of the heart and foreclose those activities. Guard of the heart is preventative and helps us anticipate rather than be a victim. Actions and their results on our hearts should be no mystery. Our hearts are sensitive to everything we do. The people we are with change us for the better or for the worse. Our environment is an energy field that either supports our way of life or challenges it. Guard of the heart is taking care of our heart and protecting it for our heart's desire.
— Mary Margaret Funk in Tools Matter for Practicing the Spiritual Life