Sit down alone and in silence. Lower your head, shut your eyes, breathe out gently and imagine yourself looking into your own heart. Carry your mind, i.e. your thoughts, from your head to your heart. As you breathe out, say, “Lord Jesus Christ, have mercy on me.” …. Try to put all other thoughts aside. Be calm, be patient, and repeat the process very frequently.
–Saint Symeon the New Theologian (949-1022)