I got this advice from a psychiatrist, who by the way, had suffered a heart attack:
"Forget your heart, completely. If it decides to stop, there is nothing you can do about it anyway. If you are lucky, someone with a defibrillator will find you in a couple of minutes.
If you are unlucky, it's your family who has the problem, not you".
Looks like horrible advice, I know. But when I think about it, there's really nothing you can do. Any worry about the heart will just cause more palpitations, chest discomfort or whatever the symptom is. Would it help to check the pulse in case of a cardiac arrest? "Oh crap, there it disappeared.. what do I do now?" Some people might say "At least I'll notice when it's happening" - which really is the only reasonable explaination. But it won't help you.