The phrase "Extraordinary claims require extraordinary proof" springs to mind.
Actually, that's a phrase I really hate, even as a skeptic- proof is proof. Yes or no, it's proved or it isn't. The "extraodinary proof" phrase really sounds like you have more than prove something, and that immediately gives the flimflam artists an excuse to say "well we could prove it but the other side always changes the goalposts"...
Any claim requires proof, end of. Not extraordinary proof because there is no ordinary or extraordinary proof - you've proved it or you haven't.
No, the phrase makes perfect sense. You have someone like John Edwards, who claims to "talk" to the dead or something. And yet all the dead he "talks" to are just loved ones with inconsequential bs messages. How about some insight into something that will really prove it. Why do we have no greater insight into the Kennedy assassination? Or any greater insight into any great historical event, with the kind of depth and detail that only someone of that era could provide?
That's the kind of evidence the phrase suggests. Again, if someone truly has "psychic" powers, then they should be able to call out the lottery, or have foreseen great events like 9/11, or the Iraq War, or the financial meltdown, etc, etc, etc....and yet all they can "predict" is the back of a card, and then only rarely and not consistently?
When there's an established, consistent track record of correct events, only then should they be given any credence. Until then, we'll just hear the excuses (goal post moving) of those making the claims.