I'll make this an answer, too.
Education. Full stop.
Here's a link. There are plenty more.
Another option, hypnosis. Some sports teams hire hypnotists to help the teams succeed. It works.
Here's a link. I have not read this one and do not know the fundamentals of the controls and so on, but have heard about sports hypnosis and its efficacy for years and years.
Meditation. This is likely down to physiological effects of improving mood and reducing stress. But the effects appear real.
Here's a link. There are others.
What do these have in common? They are integrating the functionalities of the brain.
Education is the big one.
If you do not wish to visit the links, consider this.
"What realistic methods exist for potentially dramatically increasing
human intelligence and physical athleticism, apart from nootropic
drugs or steroids?"
The answer is education. I suppose training and evolution, too. The question appears ignorant of the most basic understanding we have (and have had) of how to increase our natural abilities.
It is not through drugs, it is through usage.
The reason we are athletic and intelligent is because of evolution, and we improve upon our natural abilities through education and training.