Okido, bit of advocate of the devil here:
Don't prepare the employees!
If you've implemented your system properly, there's no necessity for preparation. I've also seen in real life that well prepared auditees (all yes/no answers, prepared files that are specifically updated for the audit, etc.) result in irritated auditors. Certifying auditors should be seen and treated as a valuable source of information and improvement.
I think preparing employees is a sign of fear of the auditor which also results in more stress on the auditees. "Aaargh, here the man I've been warned for :mg: ".
Don't prepare and let it happen, only by being open to a certifying auditor you get added value out of his audits.
edit: nope, i'm not a certifying auditor either, i really mean this ;-)