Some interesting points from Martin Fowler a few years ago on XP, patterns, design, etc. Was worth skimming through quickly.

Quoted: "enabling practices of continuous integration, testing, and refactoring, provide a new environment that makes evolutionary design plausible"

Quoted: "don't want to spend effort adding new capability that won't be needed until a future iteration. And even if the cost is zero, you still don't want to it because it increases the cost of modification even if it costs nothing to put in."

Quoted: " * Invest time in learning about patterns
* Concentrate on when to apply the pattern (not too early)
* Concentrate on how to implement the pattern in its simplest form first, then add complexity later.
* If you put a pattern in, and later realize that it isn't pulling its weight - don't be afraid to take it out again."

Quoted: "First keep in mind what you're drawing the diagrams for. The primary value is communication. Effective communication means selecting important things and neglecting the less important.
* keep them short
* don't try to address all the details (just the important ones)
* treat the resulting design as a sketch, not as a final design
"