The best way to do this is to form a mental image or picture of yourself as being well, imagining a perfectly strong and healthy body; and to spend sufficient time in contemplating this image to make it your habitual thought of yourself.
This is not so easy as it sounds; it necessitates the taking of considerable time for meditation, and not all persons have the imaging faculty well enough developed to form a distinct mental picture of themselves in a perfect or idealized body. It is much easier, as in "The Science of Getting Rich," to form a mental image of the things one wants to have; for we have seen these things, or their counterparts, and know how they look; we can picture them very easily from memory. But we have never seen ourselves in a perfect body, and a _clear_ mental image is hard to form.
It is not necessary or essential, however, to have a clear mental image of yourself as you wish to be; it is only essential to form a CONCEPTION of perfect health, and to relate yourself to it. This Conception of Health is not a mental picture of a particular thing; it is an understanding of health, and carries with it the idea of perfect functioning in every part and organ.
You may TRY to picture yourself as perfect in physique; that helps; and you MUST _think of yourself as doing everything in the manner of a perfectly strong and healthy person_. You can picture yourself as walking down the street with an erect body and a vigorous stride; you can picture yourself as doing your day's work easily and with surplus vigor, never tired or weak; you can picture in your mind how all things would be done by a person full of health and power, and you can make yourself the central figure in the picture, doing things in just that way. Never think of the ways in which weak or sickly people do things; always think of the way strong people do things. Spend your leisure time in thinking about the Strong Way, until you have a good conception of it; and always think of yourself in connection with the Strong Way of Doing Things. That is what I mean by having a Conception of Health.
In order to establish perfect functioning in every part, man does not have to study anatomy or physiology, so that he can form a mental image of each separate organ and address himself to it. He does not have to "treat" his liver, his kidneys, his stomach, or his heart. There is one Principle of Health in man, which has control over all the involuntary functions of his life; and the thought of perfect health, impressed upon this Principle, will reach each part and organ. Man's liver is not controlled by a liver-principle, his stomach by a digestive principle, and so on; the Principle of Health is One.