The mantra for focus in Hinduism is the primordial sound. When used with devotion and utmost dedication, this mantra can do wonders. It is the first sound and the sound that will remain when everything is destroyed.
The best mantra for focus is ॐ (Om).
You can add the name of your personal deity after om.
Thus the ideal mantra for focus can be Om Namah Shivaya or Om Namo Bhagavate Vasudevaya or Om Shakti Rupena or Om Gam Ganapataye.
The mantra should always be chanted in the early morning hours during sunrise.
The person chanting the mantra should take bath and sit facing northeast. He should also read the Bhagavad Gita.
Focus can be achieved only when we realize that whatever we see is nothing but manifestations of a single source of Energy.