https://en.wikipedia.org/wiki/Law_of_attraction_(New_Thought)
The law of attraction is the New Thought spiritual belief that positive or negative thoughts bring positive or negative experiences into a person's life.[1][2]
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Advocates generally combine cognitive reframing techniques with affirmations and creative visualization to replace limiting or self-destructive ("negative") thoughts with more empowered, adaptive ("positive") thoughts. A key component of the philosophy is the idea that in order to effectively change one's negative thinking patterns, one must also "feel" (through creative visualization) that the desired changes have already occurred.
Speaking from personal experience, I learned this during martial arts training. If during combat your mind is filled with thoughts of losing, it is almost certain that you will end up losing. Thus we should enter every fight believing in the existence* of a path to victory (which is in principle rarely untrue, as even someone with a massive initial advantage over you can make a sudden mistake (see below bold) which then opens up an opportunity for you) and then believing in your ability to find this path. The same applies to confrontational sports/games in general, including video games (Street Fighter, King of Fighters, etc.).
(* This does not mean feeling sure you will win, which is also to be avoided as it leads to underestimating your opponent, which is when most careless mistakes appear.)
I can attest that this approach helps, but of course it does not guarantee victory, because your opponent can also have a similar mindset, whereupon the two cancel out.
I do not believe it applies to non-confrontational activities. Telling yourself you are a musical genius does not help you compose a good song at all. If anything, it is likely to have the opposite effect, since you become less critical of your own work. Same with drawing, writing, etc..
As for physical appearance, I have noticed that people biologically age not so much according to numerical age but more according to generational shift. For example, I for a long time felt youthful even as I was getting numerically older so long as I remained in the youngest generation. But as a new youngest generation started appearing, I felt myself aging biologically just from the awareness that I now had nieces/nephews/etc.. Those of my generation who became parents themselves have biologically aged even more than I (and other non-parents) have. So perhaps what is going on is that the existence of a new youngest generation prevents the previous youngest generation from thinking of ourselves as young, and that leads to us aging more quickly. Thus reproduction is not only (most importantly) violence initiated against the child being born, but also violence initiated against non-parents in the sense of forcing us too to shift upwards in generation without any choice in the matter just because the parents decided to shift upwards in generation voluntarily.
I certainly cannot manifest worldwide non-reproduction just by thinking about it!