How should an employer prepare for a site visit?

Don’t stress. There should be no reason to panic as long as the information provided during the site visit is consistent with the information provided in the petition.
Assign a backup HR representative in case the signer of the immigration documents is unavailable.
Notify the receptionist that a DHS investigator may visit the company unannounced. The receptionist should refer the investigator to the designated person (or backup). The receptionist should not allow the investigator to speak with the employee until he or she has spoken to the designated company contact.
Notify H-1B and L-1 workers that a site visit is possible and that it is a routine procedure.
Ensure that copies of immigration related filings are readily available in case of a site visit.