It's common for competitive people to get so caught up in a negotiation that they become more committed to ending up with a deal -- even if it's a bad one. 

Remember this rule:  No one should end a negotiation worse off than when they started.  As you comfortably sit there and read this rule, you're probably saying, "Well, duh!"

Don't be fooled.  Once people get into the game of negotiating, they often adopt "the deal must be made" perspective.  Perhaps I'm preaching to the choir, but before anyone enters into a negotiation they need to formulate a place, a position or a price that is just unacceptable.  It's the point where you'll just break off negotiations and part friends.

People who routinely adopt this strategy find it really empowering.  To know that there is a KNOWN point at which you just have to walk away modifies one's attitude and their negotiating.  One's opponent will notice a difference that might even allow a settlement within the established perimeters.

One more thing.  Always break off negotiations in a gracious manner.  A curt and hostile departure gains little.  A departure with regret and even understanding will leave open the door for the discussions to be restarted without having to apologize or make a humbling concession.