Sadhu (practitioner), read Mantras deliberately and you don't meet obstacles on your path!

OM

«AUM»

Translation:

The Three Worlds: A — Material, U — Energy, M — Spiritual.
· — the Beyond-cutoff (an unpronounceable sound).

Concentration:

The Three World as manifestation of the Divine and
Turyu — the beyond-cutoff undetermined reality.

Mother`s Mantra

«Om namo Bhagavate»

Translation:

AUM — Three Worlds and the Beyond-cutoff, as a form of God.
namo — out of respect I crouch down.
Bhagavate — Supreme.

Srimad Bhagavatam 1.1.1

Concentration:

I (manifesting as the Three Worlds and the Beyond-cutoff)
completely open towards the Supreme.

Gayatri

The power of Gayatri is Light of Divine Truth. This is the Knowledge Mantra.
Gayanry-mantra is a mantra that lets bring Light of Truth to all plans of a being.
Sri Aurobindo "Letters on Yoga" Book 3, p.298

OM BHUR BHUVAH SVAH

TAT SAVITUR VARENYAM

BHARGO DEVASYA DHIMAHI

DHIYO YO NAH PRACHODAYAT

Rigveda – 3. 62

Translation:

OM — Three Worlds and the Beyond-cutoff, as a form of God.
BHUR — Bhu loka — physical plan
BHUVAH — Bhuva loka — the middle world. Prana Shakti.
SVAH — Swarga loka — Heaven.
TAT — That
SAVITUR — Spiritual Sun
VARENYAM — deserving of worship, to be welcome
BHARGO — Radiance, Spiritual Effulgence, to grant to Wisdom
DEVASYA — Devine Reality
DHIMAHI — we practice meditation
DHIYO — Buddhi — Spiritual Mind
YO — which
NAH — us
PRACHODAYAT — enlighten.

Concentration:

on the Light giving us the Godhead of Wisdom and Bliss.

Sri Aurobindo Gayatri

The Mantra that was written by Sri Aurobindo in "On Himself", p. 513.

OM TAT SAVITUR VARAM RUPAM

JYOTIH PARASYA DHIMAHI

YO NAH SATYENA DIPAYET

Translation:

OM — Three Worlds and the Beyond-cutoff, as a form of God
TAT — This
SAVITUR — Spiritual Sun
VARAM — perfect
RUPAM — a form
JYOTIH — Light, Radiance, granting Wisdom and Bliss
PARASYA — the Supreme
DHIMAHI — we practice meditation
YO — that
NAH — us
SATYENA — Truth
DIPAYET — enlightet.

Concentration:

AUM - we realize ourselves as manifestation of the Three Worlds and the Beyond-cutoff; we contemplate the Spiritual Sun that begins to manifest in us in its Perfect Form. We meditate on its Highest Light granting us Wisdom and Bliss. Let it enlighten us with the Light of its Truth.