The firmness that is accompanied by unwavering concentration, and by which one controls the activities of the mind, pranas and the senses — that firmness is of sattvic nature. (Bhagavad Gita, 18.33)
Through the discipline of constant practice one is able to give up attachment to ‘lust and gold’. That is what the Gita says. By practice one acquires uncommon power of mind. Then one doesn’t find it difficult to subdue the sense-organs and to bring anger, lust, and the like under control. Such a man behaves like a tortoise, which, once it has tucked in its limbs, never puts them out. You cannot make the tortoise put its limbs out again, though you chop it to pieces with an axe. (The Gospel of Sri Ramakrishna, 179)
Take up one idea. Make that one idea your life; think of it; dream of it; live on that idea. Let the brain, muscles, nerves, every part of your body be full of that idea, and just leave every other idea alone. This is the way to success, and this is the way great spiritual giants are produced. Others are mere talking machines. (The Complete Works of Swami Vivekananda, 1.177)
It is the nature of the mind to flit from object to object. It does not remain steady. Even in sleep it wanders about and conjures up fantastic dreams. Diverse thoughts will distract your mind and you will feel exasperated. But do not give up the spiritual struggle. It is hard to control the mind; but it must be done. There is no other way. The more your think of Bhagavan, the more the other thoughts decrease. … Through ceaseless effort one must bring the mind back if it wanders away and fix it on the Chosen Ideal. (Swami Saradananda)