OM MANI PADME HUM

According to Elizabeth Clare Prophet, in The Buddhic Essence, Ten Stages to Becoming a Buddha, page 131, the mantra Om Mani Padme Hum, which means "Hail to the jewel in the Lotus," is the "geometric formula, or mandala, that locks us in to the heart of Kuan Yin," the Goddess of Mercy and Compassion. For a brief explanation of how you can lock in to the heart of Kuan Yin with your favorite mantra, watch the following video:
For a heartwarming meditation, watch and listen to a Tibetan Children's choir sings Om Mani Padme Hum: Attribution.