My design team at Yahoo! recently defined our shared operating principles. This list of "the way we get things done" applies across all the projects we tackle and might be useful to other design teams as well.
Frame the Problem: being data-driven from the beginning enables us to understand problems before we try to solve them.
Know the Core: a common understanding of the essential essence of our products allows us to make decisions that bring them to life quickly and effectively.
Design Systems: we never focus on features in isolation and always consider the impact of any design decision on the holistic experience.
Think Outside In: putting ourselves in the shoes of our customers and colleagues gives us an objective viewpoint on global, technical, and business issues.
Stay Flexible: we know change is coming so both our approach to design and our solutions need to be iterative and adjust to where our customers, our business, and the Web is going.