You should keep your financial records accurate and up to date. It would help to verify that you have documentary evidence for all the transactions. You should reconcile your accounts to identify and resolve items before external auditors arrive. You should insist that your auditors provide a checklist of items you should be ready with before they come for external auditing.
You should also identify where your auditors can easily access your staff and the required information. Further, you will be better off if you identify key people from each department to coordinate the external audit. Finally, set an audit date that is mutually convenient for you and the auditor.