The Customer is a critical role in XP, but almost all XP practices are presented for developers by developers. While XP calls for Real Customer Involvement, it does not explain what XP Customers should do, nor how they should do it. Using Grounded Theory, we discovered eight customer practices used by successful XP teams: Customer Boot Camp, Customer’s Apprentice, Customer Pairing, and Programmer’s Holiday support the well-being and effectiveness of customers; Programmer On-site and Road shows support team and organization interactions; and Big Picture Up Front and Re-calibration support Customers steering the whole project. By adopting these processes, XP Customers and teams can work faster and more sustainably.