Ashtakshari Maha Mantra "Om Namo Narayanaya" is simple to utter, but potent in power. This is the most powerful mantra in terms meaning and sound wise.
Mantram is the one which protects them those who chants it. The ancient seers identified the oneness of universal Lord with certain sounds. They mention about one letter sound of om, two letter word of Nama and the five letter word of Narayanaya and by putting these together they come up with the mantra with eight letters I.e., Ashtakshari. This is considered to be eternal because this is the mantra that has been taught by Narayana to Nara in Badrikashram (Badrinath in lower Himalayas, India).
As mentioned in the response to question on Panchasamskaram, this mantra, is also given to the student by the Acharya along with Dvayam and Charamaslokam. Until Acharya Ramanuja, this mantra was kept as a secret and only taught by teachers to selected students or followers only. Acharya Ramanuja was the first person to change this and announce this mantra for the benefit of one and all from the rooftop of Temple Gopuram in Tirukoshtiyur located in present day Tamil Nadu, India. Therefore this Ashtakshari mantra is special for all of us as we approach the Ramanuja Sahasrabdhi.
This mantram should be read before eating anything and there are no restrictions on age, caste, creed, color and count or number of rounds of the mantra. You can chant the Dvaya mantram any time of the day and any number of times.