The December, 2017  AARP Bulletin had several articles on the Brain:
Guard your Memory - Memory Loss, Dementia Lifestyle Behaviors That Work
Avoid Alzheimer's - Memory Loss, Dementia And Alzheimer's Risk Factors 
Keep Your Focus - Attention Spans, Focus Affected By Smartphone Use
Cleanse your Brain - Sleep Is Important For Better Brain Health, Memory 

Alzheimer's Risk Factors:
Genes:
    About 25 percent of Americans have a copy of the Alzheimer's gene (APOE e4),
 tripling their risk of getting the disease.
     While the average age of an Alzheimer's diagnosis is 84 years old for people 
 without the APOE e4 gene, it strikes between eight and 16 years earlier
 for those with it.
     Keeping cholesterol under control meant a lower risk for mental decline
 in people with the Alzheimer's gene in a recent large German study. 
   
Family History:
    Having one parent, brother or sister with late-onset Alzheimer's disease
increases your risk twofold to fourfold.  
    APOE e4 gene counts or about 50%, but there are other genes involved.

Avoid head injuries

Other Risks:
Diabetes 
  Drugs pioglitazone or metformin significantly cut their risk for developing Alzheimer
  
Smoking

Microbes and gut bugs
  Herpes simplex virus 1 and Chlamydia pneumoniae may also trigger late-life brain infection.
  An unhealthy balance of gut bugs in the modern digestive system may play a role
   by increasing inflammation.
   
Some sleep aids (Diphenhydramine HCl [Benadryl, Sominex, Tylenol PM],
 Zolpidem [Ambien] or eszopiclone [Lunesta]) have been linked to dementia.
 See Sleep Aids
 
 Benzodiazepines (Xanax, Valium, Ativan, and Klonopin) linked to a increased risk of developing Alzheimer's
 | AlzheimersOrganization.org

Source: Memory Loss, Dementia And Alzheimer's Risk Factors | AARP Bulletin Dec. 2017

Livestyle factors that help your brain:

• Physical Activity - You should also get 150 minutes a week of purposeful activity -- walking briskly, playing tennis, riding your bike, swimming laps, lifting weights and the like.
• Mediterranean Diet - Eating more whole grains, fruits, vegetables, fish, nuts and olive oil and cutting back on red meat may help keep your brain in shape.
  A 2015 study "MIND diet associated with reduced incidence of Alzheimer's disease", published in "Alzheimer's & Dementia": The Journal of the Alzheimer's Association, showed that the MIND diet (Mediterranean-DASH Intervention for Neurodegenerative Delay) resulted in lower incidents of Alzheimer's disease (AD). It recommends the following 10 food groups: leafy greens, other vegetables, berries, nuts, beans, whole grains, fish, poultry, olive oil, and wine.
  See:
  The MIND diet at WebMD
  DASH (Dietary Approaches to Stop Hypertension)
  The MIND Diet Trial
• Things like Cognitive stimulating activities such a photograph classes, working with technology, genealogy research even listening to music will help.
• Having close friendships could be one key to a slower decline in cognitive function. Doing things in groups where there's a friend or coach to provide feedback seems to make new activities (like learning a language or taking up watercolor painting) even better for your brain.
  Close Friendships Linked to Sharper Memory | AARP
• Manage Blood Pressure

Myths:

• Vitamins and Supplements such as vitamin E and omega-3s have not been shown to help your memory.
• Aluminum is not likely to increase your risk. In the 70's and 80's research found aluminum in some alzheimer's brains, So aluminum pans, antiperspirants which contain aluminum were considered a risk. There is no strong evidence for this.
• Ginseng has not been shown to help.
• Brain Games may help you do better at games, but doesn't provide a general benefit.

Links:
Neural mechanisms of ageing and cognitive decline | NIH
Benzodiazepines (Xanax, Valium, Ativan, and Klonopin) linked to a significant increased risk of developing Alzheimer's
Poor Sleep Linked with Significant risk for developing Alzheimer's