The Science of Mind, by Ernest Shurtleff Holmes , at sacred-texts.com
To concentrate means to bring to a point. To concentrate the mentality means to bring the thinking to bear on one point of interest and to hold it there. Concentration has little to do
with will power. Indeed, the misuse of will often renders concentration impossible.
The simplest practice for concentration is always the best. Nature always works along the lines of least resistance; and mental power is only a force of nature and should always be thought of in a natural way.
If you wish to concentrate on some particular idea or thought, bring the attention to it; then hold it there, but without effort. At first you may find that the thought wavers; do not oppose this, but mentally brush the wrong thought aside, much as you would brush a fly from the face with the hand. Be sure that you make no great mental effort, feel at ease and at peace, gently bringing the thought back to the point of attention.
Let us illustrate this by supposing that one is going to concentrate on the thought of happiness. Taking the word "happy" into the mentality, say, "I am happy"; without effort or mentally trying, just think the words, "I am happy." In a few minutes you may find that your thought has begun to wander. Right here be sure and not bring the attention back with a bang; just say again, "I am happy," making no effort to destroy the other thought, but returning to the starting point, "I am happy." Make the whole thing easy and natural, and soon you will find that you can hold the attention as long as you desire.
It is always a mistake to oppose thoughts that interfere; when one begins to do this he will at once find that he is resisting something, thereby disrupting his whole meditation.
It is unnecessary to concentrate on an external object; for CONCENTRATION IS ALWAYS FROM WITHIN AND NEVER FROM WITHOUT. The only place that the mind can know is within itself.
In concentrating, lay aside all will power and resistance, letting the thought realize the words upon which you wish to concentrate. This will be found a simple but most effective method, and by far the most prolific of results.
In teaching a child to concentrate, it is well to have him take something in which he is particularly interested. As a child's mind wanders more or less aimlessly about, it is well to
have him write some thought on paper; and, looking at it, see how long he can center his interest on the mental picture that the words suggest.