5. Don't be too trusting
You think this contradicts my previous point? Not at all.
You hire people for the expertise; that doesn't mean they're infallible. When discussing the project with the architect or contractor, you want to come away with at least a general understanding of what they propose to do and why they want to do it. They may not have thought through important details or be unaware of critical facts. A good professional is happy to spend time going over the plans (within reason, of course) a client who's well informed now is the best insurance against misunderstandings later.