Let the other side know what your intentions are. If you tell the other side that you want to cooperate, that you care about what they want, and that to the best of your ability you are going to try to find a position that both of you can live with, then often the defensiveness and aggression of the other side will be reduced. Take an honest look at the other side's point of view. Sometimes the other side's resistance is a complete mystery to you. If that is the case, make the effort to put yourself in their place and try to see the situation from their perspective.
Successful negotiation requires finding some common ground on which everyone can agree.
If you clearly understand the other viewpoint, it is possible to see where you might agree on certain things. Make sure the other side really serious about settling the conflict. Not everybody wants to settle the conflict, sometimes they may only want to win, humiliate, or hurt the opposition. In order to change people's attitudes, it may be necessary to get them to understand that they are losing more than they are gaining by continuing the conflict. In reaching an agreement remember that an agreement will last only if everyone is satisfied that it is fair.