Drink little or no alcohol. Evidence linking heavy alcohol consumption and cancer is strong, and even greater if you smoke. Limit yourself to 2 or fewer drinks per day. For breast cancer prevention, the less alcohol, the better. The benefits of exercise extend way beyond weight control.
Tobacco is the chief cause of lung cancer and many oral cancers.
Take steps to kick the addiction now. Focus on food, not supplements. The evidence for cancer prevention is with food, not a particular nutrient or food component. Isolating a few components in a pill will not provide you with the hundred of protective benefits that real plant foods provide. Rely on real food, not supplements for best cancer prevention.