Case studies. Customer stories. According to Casey Hibbard, they:

"...serve a role that no other promotional tools truly fill by accomplishing three key purposes at once: credibility, education, and validation... Customer stories complement all other communications and bridge a gap between an organization and its prospective customers."

If you are looking for ways to sell your UX services (and who isn't these days), Casey's blog, Stories That Sell, is full of tips and tools for creating a compelling story.