It's an open strategy. You don't have to fear that the person you're negotiating with has read the same negotiating book and knows the same tricks. The method works best when all the parties involved know about it and use it. The principled-negotiation strategy consists of four parts, which the following sections relate to software schedule negotiations. Separate the people from the problem.
All negotiations involve people first, interests and positions second.
Begin by understanding the other side's position. I've seen situations in which non-technical managers had good business reasons for wanting specific deadlines. In one case, a manager felt pressure from the marketing department to produce a software product in 6 months. I told him that the best I could do was 15 months. I wish I could develop the software in 6 months, but 15 months is the best I can do.