No Need to Disagree Just to Disagree. I mean really try to get it. There are many opportunities to share different perspectives without getting into disagreements. Focus on the End, Not Just the Means. People who I've seen do consistently well with negotiations seem to take some intrinsic benefit from just being involved in the process.
Many successful people see negotiation as some sort of opportunity.
It's a chance to get to know someone better, to jointly problem solve, to resolve a conflict or to improve oneself. If It Isn't Working, Take a Break. Rather than escalate a bad situation, find a way to defuse it. Change the subject. Ask the person if he or she would like to get some coffee.