This might be ADHD-specific, but these things help:

- https://en.wikipedia.org/wiki/N-back

- meditation/mindfulness:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4403871/

Taking DHA/EPA and proteins as well as zinc might help too. There are studies conducted on this.

Also, from my personal experience: water fasting for >= 5 days (not only do you get autophagy and ketosis, you get a clearer mind too). If you are interested in this fasting thing, then I suggest consulting Valter Longo and his findings.