Your success depends on interacting well with the other person. This often seems simple and obvious, yet it requires focus and energy to truly understand what the other person is saying. See it from their view. In addition to listening, good negotiators put themselves in the other person's shoes to see the world from their perspective. Are open to persuasion.
Good negotiators can see things from the other side's perspective and acknowledge reasonable points.
Improve the Relationship. Over time, I have seen really excellent negotiators do this without being artificial or superficial. Instead, they improve the relationship by being straightforward, personal and professional. People who consistently get good results seem to just be likeable. Other people like being around them.