The Four Pillars of Alzheimer’s Prevention

Of course none of these are specific at all so you have to be your own researcher and guinea pig in trying to prevent your dementia.
http://www.dana.org/Cerebrum/2017/The_Four_Pillars_of_Alzheimers_Prevention/
Pillar 1: Diet and Supplements 
Pillar 2:  Physical and Mental Exercise
Pillar 3: Yoga/Meditation  
Pillar 4: Psychological Well-Being 
Some more details at the link.