In software development, "perfect" is a verb, not an adjective. There is no perfect process. There is no perfect design. There are no perfect stories. You can, however, perfect your process, your design, and your stories.
"Best is the enemy of good enough" suggests that mediocrity is preferable to waiting. This phrase misses the point of XP, which is excellence in software development through improvement. The cycle is to do the best you can today, striving for the awareness and understanding necessary to do better tomorrow. It doesn't mean waiting for perfection in order to begin.
In translating values to practices, the principle of improvement shows in practices that get an activity started right away but refine the results over time. The quarterly cycle is an expression of the possibility of improving long-term plans in the light of experience. Incremental design puts improvement to work by refining the design of the system. The actual design will never be a perfect reflection of the ideal, but you can strive daily to bring the two closer.
The history of software development technology shows us gradually eliminating wasted effort. For example, symbolic assemblers eliminated the wasteful tedium of translating machine instructions into physical bit encodings; "automatic programming" then eliminated the wasteful tedium of translating an abstract description of a program into assembly language; and so on up through automatic storage deallocation.
While our improved technology has eliminated waste, our increased rigidity and specialized social structures in development organizations are increasingly wasteful. The key to improvement is reconciling the two, using newfound technological efficiency to enable new, more effective social relationships. Put improvement to work by not waiting for perfection. Find a starting place, get started, and improve from there.