Find a central metaphor that's so good that everything aligns to it. Design meetings are no longer necessary, it designs itself. The metaphor should be crisp and fun.
Metaphors are a hotly debated topic in XP/Agile circles and I've never understood why. Alan Kay's quote resonates with me because when you have a good metaphor for your system, you can easily make design decisions as they arise. It keeps your design cohesive and simple. Of course, if your metaphor doesn't fit, it can have the opposite effect. I don't force metaphors, but when one pops that fits...I grab it whole-heartedly. But, I will take the time to brainstorm for one. The metaphor should be your compass that helps you navigate through your design decisions.