For one, people communicate differently in different cultures, and since communication is such a vital part of negotiation, learning to communicate effectively across the cultural divide can be a critical skill. In some culture "yes" means "I agree" while in others it merely means "I hear you." In some cultures, emotion is readily and loudly expressed, while in others it is kept close. Before you negotiate in another culture, try to talk to people who are familiar with the culture and get advice from them.
Relationships also take on different meanings and look different in different cultures, and you need to know the meaning of relationships in the culture of the other party. Similarly, the way people express and prioritize their interests, respond to solution ideas, and treat objective information also work differently in different cultures.
Before engaging in negotiations with people from another culture, try to get some information on the norms and expectations that the other party might be bringing to the table. Then use the negotiation process itself to understand what the other person values and needs, and what they expect in terms of process and communication. Remember that even within cultures, all people are individuals, and that while understanding the other person's culture serves as a general guidelines, it is even more important to listen and be receptive to learning as you progress through the negotiation.