As a licensed paramedic, I'm always annoyed by the reluctance to get involved. As was stated well above, some effort is better than no effort. If it doesn't work they are no deader than they would have been had nothing been done... but if it does work...^

That being said, I've seen some pretty strange ideas in my time from people who figure they are helping. So...
Don't throw your ice cold drink in their face... they aren't thirsty.
If they, the patient, tells you to stop, that's a good indication they don't need CPR.
Go ahead and pray if it makes you feel good, but get the heck out of the way.