"Thinking is the hardest thing to do in this world." - Gandhi
Newton didn't discover gravity by lazing under apple trees in a garden nor thinking about the, perhaps, delicious red apples. He was successful because he concentrated all his energy for some time on just one subject. That's the power of concentration - effective thinking - is all about.
Here's what Vivekananda, one of India's great spiritual gurus said more than a 100 years ago on the subject:
"To me, the very essence of education is concentration of mind, not the collecting of facts. If I had to do my education over again, and had any voice in the matter, I would not study facts at all. I would develop the power of concentration and detachment, and then with a perfect instrument I could collect facts at will."
"All the great prophets, saints, and seers of the world - what did they do? ... In one life, they perfect themselves; they have no thought for anything else, never live a moment for any other idea, and thus the way is shortened for them. This is what is meant by concentration, intensifying the power of assimilation, thus shortening the time."
"The more this power of concentration, the more knowledge is acquired, because this is the one and only method of acquiring knowledge. Even the lowest shoeblack, if he gives more concentration, will black shoes better; the cook with concentration will cook a meal all the better. In making money or in worshipping God, or in doing anything, the stronger the power of concentration, the better will that thing be done. This is the one call, the one knock, which opens the gates of nature, and lets out floods of light."
"How has all the knowledge in the world been gained but by the concentration of the powers of the mind? The world is ready to give up its secrets, if we only know how to knock, how to give it the necessary blow. The strength and force of the blow come through concentration. There is no limit to the power of the human mind. The more concentrated it is, the more power is brought to bear on one point; that is the secret."
"Free! We who cannot, for a moment, govern our own minds, nay, cannot hold our minds on a subject, focus it on a point to the exclusion of everything else, for a moment! Yet we call ourselves free. Think of it! ... The mind uncontrolled and unguided will drag us down, down, for ever - rend us, kill us; and the mind controlled and guided will save us, free us."
How can we train our mind and improve our concentration? The most effective method is meditation (there is a lot of research on meditation for the scientifically inclined).
Meditation strengthens the mind and makes it less susceptible to distraction thus improving your concentration. But before we can start meditating, it is necessary to strengthen our body. If our body is weak, our brain will keep distracting us because its priority is a fit body - it depends on a healthy body for its own survival and functioning.
Concentration is learning to train our mind to disregard any distractions. Thus the first step towards strengthening our mind is to first strengthen our body - by exercising. Once our body is fit the mind will have less to worry about its own health, and thus can be better utilised and directed by our own will. That is why, even just exercising daily will improve your concentration.
But to really strengthen it, to learn to constructively use our emotions by ensuring that they aren't uncontrollable, physical exercises aren't enough and one also needs to meditate.
Thus, the secret of a strong mind - exercise and meditate.