Bodhisattwa Avalokiteshwara (Called Chenresig in Tibetan Buddhism):
Om Mani Padme Hum
Rough Translation: 'Om, salutations to The Jewel of Consciousness (the mind) which has reached the heart's lotus.'
This mantra is practiced more than any other in the world. It is pre-eminent in producing a state of dynamic compassion in the sayer. Dynamic means that this compassion contains as part of it the ability to powerfully manifest in both subtle and obvious ways. One of the simple yet profound teachings which accompanies this mantra is the concept that when the mind and heart become united, anything is possible. The implications of this simple thought are staggering. If you want to change the world for the better, this mantra should be in your spiritual toolbox. For more on this mantra, see the chapter on the Wesak Festival.
A dedicated group of followers of Kuan Yin use this mantra to invoke her as an emanation of Avaloketeshwara, while others use the mantra Namo Kuan Shi Yin Pu Sa.
Start by picking some aspect of your life you wish to improve or some vexing problem you would like to solve or dissolve. Then pick a mantra which seems, to you, to apply. Offer a prayer to God, in whatever way you relate to God. Ask for God's blessing in accomplishing your objective in doing this spiritual discipline.
Once you have decided to undertake the discipline and offered your prayers, then pick a place where you will say your mantra for a certain number of times each day. If possible, obtain a rosary of some kind and do your mantras in some multiple of 108. If getting a rosary is not possible, then decide on a certain amount of time you will spend each day saying your mantra. It can be five minutes, ten minutes, twenty minutes or even an hour. For your first experience, any time up to twenty minutes is advisable.
The reason for taking things easy for the first 40 day commitment is due to the cumulative action of the mantra. For the first few days, all will probably go smoothly. Then as you progress, you may find that things start to get in the way of your doing the discipline: You oversleep; there is some minor emergency; you get a cold, whatever. This means that you are beginning to effect the inner 'something' for which you undertook the mantra. You are beginning to encounter inner resistance. That inner resistance manifests as outer obstacles to your discipline. It has almost become a joke in many spiritual circles in which the practice of mantra is common, that something of a very surprising nature happened on day 33 or 35 of a 40 day sadhana. It has happened to me and many others with whom I have spoken on innumerable occasions over the last 22 years.
Develop a sense of humor about it, and be thankful. There is no better indication that your efforts are working than to have small upheavals in your life while you are in the midst of a 40 day mantra discipline. Ask anyone who has undertaken one and they will have some interesting stories for you.