How to improve my negociation skill as a PM ?

You need to have a proper preparation for any negotiations you have:
Preparation for negotiation is directly proportional to the scale of the project or argument. For a small argument or discount, extensive preparation can be counterproductive. The following should, however, be considered at various levels depending on the scale of project or disagreement that needs to be resolved.

1. Goals – Why are you going into this negotiation? What do you hope to achieve and what do you think the other parties hope to achieve as well? Never analyze from your own perspective alone.

2. Available alternatives – In a situation where you are unable to reach an agreement, what alternatives are available to you? What are the advantages and set backs of the alternatives? And what are the implications of not reaching an agreement to the project and the organization as a whole?

3. Trade Advantage – What do both of you have that gives you an advantage over the other? What are you willing to sacrifice for the agreement?

4. Relationships – What kind of relationship do you have or hope to establish with the other parties involved in the negotiation? Is it a one-off relationship or a continuous relationship? What is the implication of a failed agreement on the relationship involved?

5. Power – Who has the upper hand in the relationship? Who controls more resources and who stands to gain more or lose more if an agreement is reached or not reached?

6. Consequences – For every action, there is a consequence. What are the consequences of you winning or losing the negotiation? When is it considered to have lost the negotiation and what are the quantified implications of the project and project organization?