There are no clear cut answers whether Alzheimer’s and dementia can be prevented. More large scale studies need to be done and promising research is in the works. But these habits certainly cannot hurt and can improve your quality of life in general.
Stay physically active.
Get some sun to keep Vitamin D levels up.
Control alcohol intake.
Keep your brain exercised.
Stop smoking.
Keep track of blood pressure, cholesterol and weight.
Be social!