Take a regular sheet of paper, a pen and for one minute, draw a horizontal continuous line on it.
While drawing a line, think only of it, if it occurred to you to think in a strange way, draw a vertical line on the line and start the countdown first.
The number of dashes will be your indicator, how much you know how to concentrate well.
This exercise gives an excellent effect, you will learn to concentrate for a longer time.
Once you overcome the bar in one minute, increase the performance of the exercise for another minute.
Gradually, you will accustom yourself not to be distracted in much longer periods of time.
Here is another exercise for those who do not like to draw.
Exercise to track the minute hand on the wall clock.
Gives the same effect.
During the movement of an arrow, thoughts should be, only about it.
Start again from the minute, then increase the load.