Are there ways to prevent cognitive dementia?

Meditation Practice. Here is a study (http://bit.Ly/tmwfop) done at upenn showing encouraging results in the use of a daily meditation practice as a way to prevent memory & cognitive deterioration. Also, avoid relying on your gps, iphone, google, wiki etc.= to get immediate answers to your questions. For example, use your memory retrieval function to recall stuff, even if it takes longer."use it or lose it" applies here.
No one knows. The most likely prevention strategy that we know about currently is regular exercise. If one has diabetes or high blood pressure or other chronic health conditions, it is important to keep them well controlled.
Perhaps. Being physically fit , doing regular exercise including weight training, being socially active, and learning new skills have been found to improve cognitive function. Doing puzzles is not as helpful as learning new things but may still be helpful. The better your cardiovascular system works, the better your brain functions. Having enough sleep and a healthy emotional state are essential.