Rules of thumb? Don't make mental mistakes. To be honest, I don't always win that fight. My gun room is littered with scorecards that have a single miss highlighted by my handwritten notation "I missed because I was being stupid"
Rule #1 - There is always somebody at the match that knows more than you do.
Rule # 2 - You can't fix the last shot, only the next.
Rule # 3- Learn something- see rule # 1
Don't change your stage plan based on the run just before you! (Especially if you are running single stack or production and they are running limited or open)