don't know if this helps..... but try to think of yourself as more of a professional than a private pilot. Think about your need for knowledge of systems, weather, contingencies, etc. Don't over do it, but try to come off as being serious and just...well....professional - knowing your stuff, but not being arrogant. If you make a mistake, admit it and correct for it.
If you feel that you honestly aren't ready for your commercial ride, don't take it! Remember that part of being a professional pilot is trying not to put yourself in a compromising situation...especially right off the bat.