To obtain magical power, one must strengthen the will. Let there be no confusion between will and desire. You cannot will too strongly, so do not attempt to will two things at once, and while willing one thing do not desire others.
Example: You may at times have passed a person in the street, and as soon as passed may have felt some attraction, and the will to see him again; turning round (you) may have found that he also turned to you. The will, although untrained, may have alone done this. But if you, untrained, walk out again, and decide to make the experiment of Willing that he who passes you shall turn round, and try it, you will fail. Because the desire of gratifying your curiosity has weakened the force of your will.