Negotiators must overcome barriers to effective communication.
While a deep, fast moving river between you and a hungry lion may appear to be an effective barrier, a locked cage with strong bars would be even better.
To be effective a negotiator must have his message clearly heard and understood by the other person. Barriers to effective communication can obscure the best argument. Look for and remove barriers that block your message. All negotiators must be, by definition, skilled communicators. That means they must listen as well as speak clearly.
But there is more.
Reactions to what you are saying signal if the other person is listening and understanding your message. Watch the listener's eyes. If they stay focused on your eyes, that usually means they are intently listening. If however, the wander or disconnect, it usually means that their mind is racing ahead to formulate what they are going to say, that they are not believing what you are saying, or that they are thinking about the hot date they have that evening. In any event, you need to regain their attention. An effective way to do this is to simply stop speaking. When they realize that you are no longer speaking resume as though nothing has happened.
You may actually need to call their attention to the fact that they weren't listening and ask why. This tactic will often uncover the reason for the barrier. Then it can be properly addressed. For example, I recall a situation where the other person replied, "I'm really sorry, my daughter is very sick and I'm distracted." I said that I was sorry to hear about her daughter and suggested we put off further discussions until she was better. In recognizing her personal need, we dealt with each other as real people. Later this personal respect helped us to overcome some of the tougher issues we were facing.
Unless communications are being heard, they should be forestalled until the other person is able to hear what you have to say.
The mere act of acknowledging barriers to communications can give you the opportunity to work together to start to agree on how to resolve the barriers. Then it will be easier to discuss and resolve the real issues.