This is a very good article by Gyan Rajhans 

"Mananaat traayate iti mantrah"
(That which uplifts by constant repetition is a Mantra.)

Sound Is Power
I firmly believe that the sound of Mantra can lift the believer towards the higher self. These sound elements of Sanskrit language are permanent entities and are of everlasting significance. In the recitation of Sanskrit Mantras the sound is very important, for it can bring transformation in you while leading you to power and strength.

Different sounds have different effects on human psyche. If a soft sound of wind rustling through leaves soothes our nerves, the musical note of running stream enchants our heart, thunders may cause awe and fear.

The sacred utterances or chanting of Sanskrit Mantras provide us with the power to attain our goals and lift ourselves from the ordinary to the higher level of consciousness. They give us the power to cure diseases; ward off evils; gain wealth; acquire supernatural powers; worship a deity for exalted communion and for attaining blissful state and attain liberation.

Origin of Mantras
Mantras are Vedic in origin. The teachings of the Vedas consist of various Mantric chants or hymns cognized by different seers or Rishis from the Cosmic Mind. Since the Vedas are impersonal and eternal, the exact historical date of the origin of Mantra chanting is hard to arrive at. For example, every Mantra in the Vedas, Upanishads and various religious traditions (sampradayas) within Hindu religion begin with Om or Aum - the primordial sound, the sound that is said to have its origins at the time of the creation of the cosmos - also referred to as the 'Big Bang'.

Om: The Beginning & the End
The Bible (John 1:1) says: "In the beginning was the Word and the Word was with God and the Word was God." Modern Vedic philosophers have interpreted this teaching of the Bible, and equated Om with God. Om is the most important of all mantras. All mantras generally begin and often also end with Om.

Healing by Mantropathy
The chanting of Om in Transcendental Meditation has now received widespread recognition. Mantras can be used to treat tension and many other difficult diseases that are yet to come. The Brahmvarchas Shodh Sansthan, research center for integration of science and spirituality in Shantikunj, Haridwar, India, is the only place I know of which carries out extensive experiments on 'mantra shakti'. The result of these experiments is used to testify that Mantropathy can be used scientifically for healing and environment cleansing.

Over the last 21 years of my Vedic religion broadcast, several listeners have told me how they have benefited physically and spiritually from chanting Maha-Mrtyunjay mantra for 15 minutes every morning.

How to Chant
There are many schools of thought on the methods of chanting. A Mantra chanted correctly or incorrectly, knowingly or unknowingly, carefully or carelessly, is sure to bear the desired result for physical and mental well being. It is also believed by many that the glory of Mantra chanting cannot be established through reasoning and intellect. It can be experienced or realized only through devotion, faith and constant repetition of the Mantra.

According to some scholars, Mantra chanting is Mantra Yoga. The simple yet powerful Mantra, Om or Aum harmonizes the physical forces with the emotional forces with the intellectual forces. When this happens, you begin to feel like a complete being - mentally and physically. But this process is very slow and requires a lot of patience and unfailing faith.

The Guru-Mantra
In my opinion the healing through chanting can be expedited if the mantra is received from a guru. A guru adds a divine potency to the mantra. It becomes more effective and thus helps the chanter in his/her healing faster.

My Personal Experience
Now, let me give you my concerted opinion based on over two decades of chanting "Om Gam Ganapatayae Namah", the Mantra given by my Guru. It has warded off all evil and blessed me with abundance, prudence and success in every walk of life. Moreover, when I chanted this Manta before beginning a journey, a new job, or before entering into any new contract or business, all impediments were removed and my endeavors were crowned with success. The credit of all my worldly and spiritual successes goes to my Guru-Mantra 'Sadhana' - the complete faith and adherence in the mantra given by my Guru.

Keep the Faith!
It is important to have complete faith in the recitation of Mantras. It is primarily through faith - aided by strong will - that one achieves one's goals. A sound body and calm mind are essential for the chanter of Mantras. Once you are free from all worries and have achieved stability in mind and body, you will derive maximum benefit through the recitation of Mantras. You must have a definite object in view and a strong will power to obtain the desired objective, and then direct that will to achieve the goal.

http://hinduism.about.com/od/prayersmantras/a/mantrachanting.htm