Keeping cholesterol levels very low (LDL cholesterol less than 100 mgs) by using statins, keeping BP below 120/80 mms Hg, maintaining an ideal body weight, regular exercise programme, eating heart healthy diet rich in fruits and vegetables and keeping away from tobacco are very effective means of preventing heart attacks.
A normal ECG is good and indicates absence of any injury to the heart but does not rule out blocks in the coronary arteries. A stress ECG or a stress echcardiography is a much better prognostic test, especially in high risk individuals like you.