I tell people to start implementing when they are pretty sure there aren't more important stories out there. An iteration's worth of data is worth months of speculation.
Kent BeckThe system metaphor is a story that everyone--customers, programmers, and managers--can tell about how the system works.
Kent BeckResponsible Development shares many practices with XP but the roots are different. Responsible Development's values are honesty, transparency, accountability and responsibility. These lead me to pairing, test-first, incremental design, continuous integration and so on because they support the values.
Kent Beck