to the following:

1) Adequate sleep/circadian rhythm - our brain cleans itself while we are sleeping and rejuvenates itself
2) Good Nutrition - what we eats affects us and to name just one, we eat between 120-130 lbs of sugar per person per year... These are empty calories that strip our body of needed supplies of vitamin B (specifically thiamine, niacin and biotin). Vitamin B is critical to proper production of serotonin and other neurotransmitters.
3) Exercise.

That being said there are still many mental illnesses that will afflict people. But many who suffer could have their symptoms alleviated or cured by just following the above 3.