In negotiations, you'll often have issues you can't seem to resolve, even though you jawbone them to death. At such times, don't be afraid to agree to disagree. Unless it's an obvious dealbreaker, consider putting touchy points aside until after you've settled the easy ones. Flexibility, compromise and creative solutions come more easily once each side trusts and understands the other a little better.
When all you can say is no, say it diplomatically. Give a plausible explanation for why you can't concede a point, not a crappy, high-handed or arbitrary one. Respect opponents' intelligence and they won't resent you. The explanation may also help them save face with their colleagues.
And at the very least, hear them out. Your hands might be tied for a variety of reasons. You may have to send the other side back empty-handed. But if you give them a few minutes to vent, complain or protest, they'll feel better. Let them whine a little bit, and (maybe) they won't tell everybody that you're an ogre.
On the other hand, being difficult, terse and sour has strategic advantages. I once worked for someone whose favorite cartoon was a picture of a gruff executive poised over a handheld buzzer attached to a huge scoreboard filled with lightbulbs spelling out the word "NO." The caption read, "You can run it by me, but I'm pretty sure I know what the answer's going to be."
Truly, the leathery countenance that says "don't even think of asking" can discourage even the sturdiest opponent. And a flat no is often the best way to quash inappropriate or idiotic demands. Just remember that being known by many expletives only works while you're in the one-up position. At low points in a career, such a reputation is a double-edged sword. There's a big difference between being savvy and tough, and simply being a jerk.