Namely, coughing can provide a great help. Namely, you should cough energetically and continuously as you need to cough out some secretion and you should breathe in deeply.
Deep breaths will help you intake more oxygen, and coughing will cause a pressure on the heart and maintain circulation and a normal heart beat.
The breathing in and coughing should be repeated every two seconds continuously until some help is given to you by a medical person or the ambulance.
In case you already feel some improvement, wait for a while and then stop coughing. In this way, people which have just experienced a heart attack can get some time to go to the hospital and avoid severe consequences.