I passed my CPL in November and still remember the nerves and anxiousness I felt days before the test. Just to reiterate what you've been told here already, you would not have been recommended for the test if you weren't up to speed. Try and relax and be confident in your ability. Don't let the ATO intimidate you. Be respectful but not over-awed. They want you to pass as much as you do.
You should have a rough idea of which area the NAV might take you. One tip I received was to mark on your WAC the runway directions, frequency and elevation of all the little strips in your area that you might be suddenly called on to find for a diversion or PFL. Saves rummaging around for the ERSA or CAG. Know the aircraft inside out as I'm sure you are aware of. You may/will be asked about the location or function of little random things that perhaps you have never thought much about.
Get there early and prep the a/c as soon as you can. I fuelled up the night before and cleaned all the windows. Just something less to stress about. And because the NAV is a mock charter, don't be afraid to pack water and snacks for the 'pax' as someone mentioned. I gave them a newspaper too, which they actually read. And one thing I would suggest is to make sure you're fluent and competent in you pre-flight pax brief. If it flows well and sounds professional then your flight should begin the right way.
Above all I think is to learn/refresh as much as you can. The better you answer the ground questions, the more confident you feel and the higher the standard of flying. It all seems to gel when you get off on the right foot.
Good luck.