Remember, negotiations are a dynamic process and nothing need be written in stone. They won't tell you after you start bargaining. Poor concessions are often made only when the deadline looms. So find out your opponent's deadline, and conceal yours. Stick at it beyond the point of boredom.
Power is based on perception.
If your opponent thinks you are more powerful, you are. So act as though you are. In spite of the above, be ready to ask for help. Your opponent will usually respond favorably, and it will make the atmosphere more friendly. That is, think about broadening the pie , it is very rare that the pie which is to be divided cannot be made bigger so that all the parties get a good piece of it.