Note that there is a difference between being assertive and being aggressive. You are assertive when you take care of your own interests while maintaining respect for the interests of others. When you see to your own interests with a lack of regard for other people's interests, you are aggressive. Being assertive is part of negotiation consciousness. Become a good listener.
Negotiators are detectives.
They ask probing questions and then shut up. Many conflicts can be resolved easily if we learn how to listen. The catch is that listening is the forgotten art. We are so busy making sure that people hear what we have to say that we forget to listen. You can become an effective listener by allowing the other person to do most of the talking.