Here’s an interesting quote by Prentice Mulford:

“A principal means for holding and increasing both physical and mental strength lies in the training of the mind and body to do but one thing at a time; in other words, to put all the thought necessary for the performance of any act in that act, and to put aside all other thought whatever, save what belongs to that act.

Every physical act costs a thought, and every thought costs a certain outlay of force. Every step you take involves a plan to give that step direction. Plan involves outlay of thought. Thought means outlay of force. If you think of other things while walking, you are expending force in two directions at once.

Do you think that an acrobat could so readily ascend a rope hand over hand, did he not put his whole mind as well as strength on the act? or that an orator could thrill an audience, were he obliged to turn a grindstone while speaking? Yet, in so many of our acts, do we not unconsciously burden ourselves by turning that grindstone, in thinking and planning one thing, while doing or trying to do another?

If you are going up a hill and are continually looking with impatience toward the top, and wishing you were at the top, you will soon become tired. If you are near that hilltop in imagination, while your body is near the bottom, you are sending your force of thought to the top of the hill, leaving only enough in the poor, outraged body to drag it wearily upward.

If you hold all that force to that body, and concentrate it on each step, you ascend far easier; because your power is then concentrated in those parts of your body (your legs) that most need that power. When you concentrate all your strength in each step, you make each step easier, you get a certain pleasure out of each step, and you forget all your trouble – that being the impatient desire of being at the hilltop.

This law holds good in every act of life. Do you not wish you could forget your trouble, your disappointment, your sense of loss, through concentrating all your thought on something else, and becoming so absorbed in it, and enjoying it, as to forget all things else?

This is a possibility of mind, and is one well worth the striving for. It can be attained by the practice of concentration.”

Start your concentration training with the course “Concentration and Mind Control”