There are two categories of mantras delineated in the shastras.
One is the vaidika-mantra, or mantras of the Vedas (such as Gayatri, etc.), and the other is the nama-mantra, or mantras composed of the Lord's names. By definition a vaidika-mantra will possess three aspects, namely a seed letter (such as 'om', 'srim', 'hrim', etc.) which invokes the divinity, a deity to whom the mantra is directed, and a function or offering to the deity (such as 'namah', 'svaha',etc.). Such vaidika-mantras must be received directly from the guru through the process of upadesha, or initiation. This category of mantra has many rules and regulations associated with the chanting of the mantra. It is not that one may chant the mantra at any time or at any place.
Prior to the chanting of the mantra one must undergo purification both externally and internally. This involves bathing (and other procedures from the 'yama' and 'niyama'), meditational postures (asana), breath control to make the mind peaceful (pranayama), withdrawal of the senses from the sense objects (pratyahara), and absorption on the deity (dharana).
These six steps of yoga help one to purify oneself, first externally, and then internally. Once situated in purity one may then begin one's mantra sadhana, provided one finds a suitably pure and peaceful location.
Such mantras require one to be completely dedicated to their sadhana under the guidance of a qualified guru.
These mantras should not be chanted whimsically, as there is all chance of committing mistakes in our sadhana, resulting in us incuring negative reactions.
Nama-mantras are quite different from the vaidika-mantras, as they are composed only of the Lord's name.
Such mantras carry the exact same potency as the vaidika mantras, but in a manner accessable to all.
When we analyse the power source of all mantras, we will come to understand it is the name alone which empowers the mantra.
For example, one may take a mantra such as 'om namo bhagavate vasudevaya'. If one were to just chant 'om namo, om namo, om namo', one will not receive the effect of the mantra. On the other hand, if one were to just chant 'vasudeva, vasudeva, vasudeva', one would receive the benefit of the mantra, for it is Vasudeva's name which empowers the mantra.
We find countless examples in the Puranas o people calling out the name of the Lord and receiving His full anugraha, His merciful glance, despite not being trained in Vedic mantras.
Draupadi was saved by Lord Krishna in the assembly of the Kurus simply by invoking His divine name.
Similarly, Ajamila was saved from the messengers of Yama simply by calling Narayana at the time of death.
The great rishi, Valmiki, was previously a sinful hunter, but by chanting the name of Rama he was transformed into agreat saint.
The power of the Lord's name has been demonstrated through countless histories as recorded in the ancient Puranas.
The full potency of the Lord is present within His holy name:There are no restrictions (niyamas) for chanting the Lord's name; neither must one take consideration of the time or circumstances.
"The nama-mantras are the Lord's special mercy to the fallen souls of the Kali-yuga.
Those of us with no proper spiritual qualification can still attain the Lord and associate with Him through His divine name. Whatever result was obtained in Satya-yuga by meditating on Vishnu,in Treta-yuga by performing sacrifices, and in Dvapara-yuga by serving the Lord's lotus feet can be obtained in Kali-yuga simply by chanting the names of Sri Hari.
There are three ways by which one can do Japam
Ucchahi - loud recitation - less effective
Upamshu - soft recitation - more effective
Manasikam - within the mind - most effective.
Benefits of Japam
Physical - to energize the system (to tone up the system)
Mental - to remove depression/obstacles/mental blocks.
Spiritual - self-development/specific areas of growth (strengthening the inner-personality by projecting positive powers inwards; over-coming/eliminating negative traits)
‘Mantra Sastra’ suggests Sahasranama japa for achieving the odds and ends of worldly life.
If chanting the entire Vishnu Sahasranama is difficult then only part of it that corresponds our Nakshatra [from below] can be chanted.
Ashwani (Aswini)1 to 4
Bharani 5 to 8
Kruttika (Krithikai)9 to 12
Rohini 13 to 16
Mrugasira (Mirugaseersham)17 to 20
Ardra (Thirivathirai) 21 to 24
Punarvasu (Punarpusam)25 to 28
Pushyam (Pusam) 29 to 32
Aslesha (Ayilyam) 33 to 36
Makam37 to 40
Poorvapalkuni (Puram) 41 to 44
Uttaraphalguna (Uthiram) 45 to 48
Hasta (Hastham) 49 to 52
Chitra (Chithirai) 53 to 56
Swati (Swathi) 57 to 60
Visaakhaa (Visakam) 61 to 64
Anuradha (Anusham) 65 to 68
Jyeshtha (Kettai) 69 to 72
Moolaa (Mulam) 73 to 76
Poorvaashaadaa (Puradam) 77 to 80
Uthraashaadaa (Thiradam) 81 to 84
Sravanam (Thiruvonam) 85 to 88
Dhanishtha (Avittam) 89 to 92
Sathabishang (Sathayam) 93 to 96
Poorvaashaadaa (Poorattathi) 97 to 100
Uttarabhadra (Uthirattathi) 101 to 104
Revati (Revathi) 105 to 108
A person not sure of one's Nakshatra can chant all the 108 slokhas and get rid of all ills associated with one's Nakshatram and attain sampoorna saanthi.
Srivishnusahasranaama Stothram removes all the ills associated with any grahadoshams to any chanter of any Nakshatram and provides supreme solace and supreme bliss.
Even nama japa of Hare Rama Hare Krishna Krishna Krishna Hare hare, provides the benefits of Sahasranama, if done with great dedication as is said in the Phala Struti of Vishnu Sahasranama……..
Shri Rama Ram Ramethi Rame Raame manorame,
Saharanama Thatulyam Rama Nama Varanane