Capturing design rationale

How many times have you worked on a long-term project and the design rationale gets questioned by various stakeholders at late stages of the design and development process?

This has happened to me many times, and most of the time I am madly scrambling back to emails and meeting notes trying to find who approved something and why. Most of the time this search ends in futility, thus sparking a new design debate on something that had already been finalized.

My question is: how do you guys capture design rationale, and how specific is it? (for example, saying that “Ken X in Marketing approved this approach on Jan 7, 2003 at the status meeting”.