1. Smoking Cigarettes
(Spencer Platt/Getty Images)
The good news is that fewer of us smoke today than ever before. The bad news: Young people are still taking up the habit, and it's the single most important risk factor for heart disease in those under 50. Smoking increases blood pressure and the risk of blood clots. It also decreases exercise tolerance and "good" HDL cholesterol. Women who smoke and use birth control pills have a much greater risk of stroke.