I have been both a mediator and advocate in negotiations for many years. In addition to preparing, listening, understanding the importance of both the time spent in and the timing of negotiation, and being pleasant, I can not think of anything more important than resisting negative urges when you are negotiating. What do I mean by a 'negative' urge? Sometimes, I refer to them as counter-productive urges. Whatever names they go by, they are things you want to do that are more likely to impede progress in response to something someone else has done that you do not like.
What are some examples? The most common is assuming a "like kind" posture in negotiation when the other side does something that you do not receive positively. An unrealistically high start followed by an unrealistically small response; a frustratingly small move begetting a frustratingly small response; a perceived reversal of coarse prompting a move in the opposite direction; and dueling ultimatums are all examples of things that are seen or threatened in negotiations.
Reasons to avoid them should be obvious. The frequency with which they manifest themselves suggests that the reasons are not appreciated to the degree that they should be. While every case, participant, and context may be different, generally speaking, following through with a negative urge can lead to the unintended consequence of bogging down the process of reaching an eventual deal, or worse yet, derailing it.
We are all human. We are both intellectual and emotional beings. The emotional side is speaking loud and clear when we have an urge. When you feel one coming on, be mindful of unintended consequences, take a deep breath or pause, and think about how you can address your concerns in a less reactionary way that keeps you on course to your target. To do otherwise may just play into the hands of the other side.