Sri Chinmoy: To still the mind for concentration, you have to use your willpower. If your concentration is so dispersed that you cannot bring the mind under control at all, there is a particular exercise which you can practise.
Make a small black dot on the wall and stand six or seven feet away, focusing your attention on it. Try not to see the white wall around you. See only the black dot. After five or ten minutes try to enter into that dot and feel that you have become the dot itself. You are not something else or someone else. You are inside that dot. Finally, pierce the wall with your focus of concentration. Go right through the wall and beyond it. From that point, look back and try to see your original self, the person who was standing in front of the wall looking at the dot. Once you are successful with this exercise, the mind can never act like a naughty child. At that time, the mind becomes a most faithful servant of the soul and concentration becomes quite easy.