Just did a basic CPR course and it was obvious that speed of help is the key. This means getting to a defrib asap and not being afraid to use it. It is pretty simple and the machine will talk you through it.
BUT It does mean you need to know where the nearest one is. 999 or 112 will know based on where you are, but seconds count.
As you go around your normal areas keep an eye out for the yellow boxes, then you can shave those seconds off and maybe save a life.