Any kind of Cardiovascular Exercise, which you should participate in 3-4 times per week, for 20-30 minutes continuously. Your Heartrate needs to get to at least 65% of your maximum capacity.
Eating less saturated fats is important too, to prevent Arteries clogging up. Eat oily fish about 3 times per week or take Cod Liver Oil supplements daily, reuduce Alcohol intake and don't smoke.
If you do these things, then you have a pretty good chance of maintaining a healthy heart for life, providing you have no congenital defects in the first place.