As far as manifestation goes I think the concept of the ‘tulpa’ is the epitome of physical manifestation.

A tulpa, is an entity created by an act of imagination, rather like the fictional characters of a novelist, except that tulpas are not written down. It involves intense concentration and visualization. It can grow from first being a mere subjective entity, then slowly to a imaginary ghost like quality and finally becoming indistinguishable from objective physical reality.

As far as manifestation to accomplish a goal in life, or to perfect a task, professional athletes do it all the time (visualization, which is a form of manifestation) to ‘perfect’ and hone their skills to their ideal. The mind and imagination are among the most powerful tools available to man, much more powerful than a Pentium 4 or the internets. The quickest way to become the idealistic perfect person you always want to be, is to imagine in your mind with absolute exact clarity and vividness just WHO that person is, capture the essence and soul of the person, feel the energy of the person and his/her radiance and expression. You will become that person, that person is your higher true self.

Visualization can also be used to cure diseases, or to heal a certain part of your body. Don’t be skeptical or it won’t work, faith and belief often changes a lot of things by (apparent) magic.

I think manifestation/visualization/guided imagery works something like this.
There definitely is an effect, even if it’s only emotion that affects people too.
(Otherwise evil drug giants that make Zoloft and other mood artificial enhances would be out of business)