Requirements Negotiation: When, How, and Who?

I'm trying to figure out what works and what doesn't. Hoping you will reply and share your experiences adressing these questions:
1) When do you use negotiations? What part of the SW life cycle, or what events trigger it?
2) What particular methods do you use and what, if any, tools are used?
3) How is the process organized? Who is involved? Who leads? What are the artifacts?
4) How successful has this been? How do you determined success (lack of success) of the implemented process?

I've read about the WinWin Model, and requirements triage. But I want to know what people/organizations are actually doing - an in the trenches perspective.