Here are a dozen techniques I try to practice in every negotiation. This is not just the motto of the Boy Scouts.
Preparation is the single most important element in successful negotiations.
In negotiations, information is power. The more relevant information you have, the better your position is. Preparation for your negotiations can not be overdone. Allow yourself adequate time to prepare prior entering any negotiation. Your relationship with this party may not normally be described as adversarial, for the purposes of this discussion we will view the negotiation as an adversarial relationship.