One of the best quotes I have come across was from Robert C. Martin. For me this quote reemphasizes the ideal to keep things simple and stop when a concept is working, simple and explicit. It might not be perfect, but some times OK, is good enough.
No matter how much time you spend trying to figure out the perfect software structure, you will always find that the customer will introduce a change that violates that structure. There is no escape from this. There are no perfect structures. There are only structures that try to balance the current costs and benefits. Over time those structures must change as the requirements of the system change. The trick to managing that change is to keep the system as simple and flexible as possible. Robert C. Martin (Agile Software Development, page 325)
Looking back at this quote today, it still rings very true.