[fjabre]

Magnesium citrate before bedtime.

It will improve your sleep quality tremendously which will then improve your concentration naturally. A rested mind has an easier time getting into the zone.

Buddhism and Zen can also help. No need to become a master, simply understanding and applying some of the basic concepts can help mind state significantly. This can also boost concentration by quieting the mind.

Also check out:

TMS Therapy or Transcranial Magnetic Stimulation. It is brain stimulation technology. Very interesting and lots of research behind it on pubmed.