If you are over 40 (or have a history of dementia/cardiovascular problems in your family) you should get your cholesterol levels checked regularly. Managing your cholesterol by eating a balanced diet that avoids saturated fats will also help. Find out about managing cholesterol
If you are over 40 (or have a history of dementia/cardiovascular problems in your family) you should get regular blood pressure checks to ensure it is at recommended levels. Find out about managing blood pressure
Adopt a balanced diet for life. It's fine to treat yourself to the occasional slice of cake but try and eat healthily and moderate your fat intake most of the time. A Mediterranean style diet may help reduce risk and is relatively easy to follow. Find out more about healthy eating
Overall, try and lead an active lifestyle that combines physical, social and mental activity. Not only will it help to reduce your risk of dementia, you'll hopefully enjoy life more as a result!