>I'm a proponent of Top-Down design (which may help further). I think it fits well with OOP principles.

Actually, I've lost the "top" ... I'm working more from a component-composition point of view ... with everything interacting pretty well on the same plane. I like to think of it as one big "mechanism" with no single driver.

If by top-down, you mean class hierarchies ... then yes. But if I'm working at level n and realize I could use a new property or method in the base that I can inherit from, I add then and there (to the base). It's not too long before I have all my class libraries open (in my editor).