Having an architect can be helpful but it can also be significantly more expensive and if there is a problem, then you are likely to be pulled into the argument anyway.

Definitely communication, good diagrams, developing a good rappour with the contractor and not being overly tight or strict with them otherwise you can build resentment. For example, don't withhold all of their expected money because of a single piece of dodgy drywall, give them most or all of the money and make sure they make-good first thing next week.