If you're confident before you negotiate, a little physical exertion can make you more effective. But if you're nervous, you might want to try this.
"If you're sweating and your heart rate is up, it's seen as a sign something is going wrong, that you're too nervous, off balance, flustered," M.I.T. associate professor Jared Curhan tells the New York Times. "Whereas we're showing that something could be very right."