I tell them: "We both know that how much I am currently paid has absolutely nothing [emphasis in tone] to do with how you decide to compensate me."

This lets them know two things: 1. You're not going to divulge your current pay no matter what, and 2. Gives them a face-saving 'out' by complimenting them that their company is "different" and special (and makes it sound as if your compensation is totally their decision; the negotiation over a number has not begun yet).

There's also the anchoring theory, where the discussion centres around the first number that any party says out loud. So in the case where the company goes first they might offer a low number (for them) which may be reasonable for you but not what you want.

So the way to use that to your advantage is to say a number first, but make sure it is far more than what you think you can get, but not absurdly high. That way if they accept you'll be pleasantly surprised, but if they negotiate down it will be around that figure.

Of course if you're not confident in that, just let them put out the first number and go from there. But NEVER tell them what you are currently getting, that will anchor your salary to what it is now plus a small increase.

"I'm sorry but my employement contract with my current employer doesn't let me talk about my current salary while I'm employed. You wouldn't want to employ someone who breaks contracts, would you?"