One way to encourage cooperation in negotiations is to enhance this repetitive aspect. Negotiations can be broken down into a number of stages. Parties may have to deal with each other for a long time. Or concern for one's reputation may link behavior in one set of negotiations to other negotiations. In real life, negotiators have two advantages over the more formalized situation of the Prisoner's Dilemma.
Parties can communicate with each other, and they can make binding commitments.
Parties can communicate their intentions, and so reduce the uncertainty which makes competition seem like an attractive option. In real negotiations there are also a number of ways to make cooperation seem more attractive than competitive value-claiming behavior. Focusing on interests rather than positions encourages a cooperative approach. Negotiators can build cooperative momentum by exhibiting a strong early commitment to cooperative attitudes. A particular negotiation may be located within a larger relationship and an ongoing series of dealings.