The most important treatable conditions linked to stroke are:

High blood pressure. Treat it. Eat a balanced diet, maintain a healthy weight, and exercise to reduce blood pressure. Drugs are also available.
Cigarette smoking. Quit. Medical help is available to help quit.
Heart disease. Manage it. Your doctor can treat your heart disease and may prescribe medication to help prevent the formation of clots. If you are over 50, NINDS scientists believe you and your doctor should make a decision about aspirin therapy.
Diabetes. Control it. Treatment can delay complications that increase the risk of stroke.
Transient ischemic attacks (TIAs). Seek help. TIAs are small strokes that last only for a few minutes or hours. They should never be ignored and can be treated with drugs or surgery. Learn more:

http://www.ninds.nih.gov/disorders/stroke/stroke_bookmark.htm