What lifestyle choices are helpful in reducing the risk of or slowing cognitive decline?  

Regular daily physical exercise. A healthy diet: for example, the MIND diet: low in fat and sugar, high in vegetables and fruits, and regular hydration. Be mentally active. Participate in social activities. Get adequate sleep (7-8 hours nightly). Manage stress. Limit alcohol and avoid smokin...

The same that is needed for preventative general health, there are no evidence-based lifestyle measures unique to cognitive impairment.  Here is a fact sheet with citations from the Physicians Committee For Responsible Medicine.