An easy way to keep the heart (and the whole body, for that matter) healthy is to exercise. There's no way around it. You need to get active.
Walking, running, or swimming are great forms of cardiovadcular exercise. Playing sports or riding a bike are other great ways.
Most forms of cardio get the heart working hard, and thus should also be considered.
Weight lifting also gets the heart pumping, but it is more useful for muscle training.
Sitting down for long periods of time is not good for the body.
Sitting can lead to a condition called deep vein thrombosis, which is when blood clots form in the deep veins of the body, usually in the legs.
Being overweight is not good for the heart. Being overweight causes all sorts of problems such as high blood pressure and diabetes.
For most people, BMI is a useful indicator of what proper weight should be, and it is recommended to use it.
However, if you lift weights, are a body builder, or have large muscles in general, The BMI is not very helpful, because it does not take muscle mass into account.
A good example is sumo wrestlers. They are very large and overweight, but they also have a level of health greater than the average person.
Keeping the blood pressure, and blood sugar levels normal is important too.
On the Diet
High cholesterol is not good for the body, and should be limited in intake. The USDA recommends no more than 300mg of cholesterol be consumed a day.
Not all fat is bad. The body needs some fat as fuel, but certain types of fat are bad for the body.
Saturated fat and trans fat should be limited. Pay attention to the daily value on food labels.
In a similar vein, not all carbohydrates are bad. The body needs some carbs as fuel, but one should take care not to eat too many.
There is a difference between complex and simple carbs. Simple carbs provide quick energy, but are not as good for the body.
Complex carbs, on the other hand, take longer to break down longer, and keep you full longer.
Some sources of simple carbs are white breads, pastries, and sugar.
Some sources of complex carbs are whole grains, and vegetables.
Some foods that are great for the heart are nuts and fish (foods high in omega-3's and omega-6's).
Feelings like anger and stress are not good for the heart (or body) either. Taking some time to relax, and trying to manage anger are steps to take.
Not to say that these feelings are bad. It's natural to be angry at things, and there are plenty of healthy ways to cool off.
People with anger issues however, should seek ways to manage their anger, and anger should not be taken out on friends and family.
Unless you are venting. Finding someone to vent to, or just having someone willing to listen to your problems is another healthy way to reduce anger.
Some last tips for heart health, which apply to the entire body too.
Getting the proper amount of sleep can help reduce heart attack, and sleep is generally important for the health of the entire body
STOP SMOKING (if you do).If you live with people who smoke, strongly encourage them to stop too.
There are no benefits to smoking. It ruins your health, damages your body (and wallet), makes you smell and look bad, and shortens your life.
If you don't smoke, then avoiding second hand smoke is important too. Living with people who smoke can also damage the body.