Professional, have copies of everything they are asking for that you can provide. Don't give the examiner the originals, let them review them and provide copies to for them to keep. If they ask you a question you don't know the answer to don't be afraid to tell them you don't know, but will find out and get back with them. And then follow through. Unless the client has been summonsed to be there it's probably a good idea to not have them in the meeting. Don't bring anything to the meeting you are not prepared to turn over. If you are also the original preparer you need to make sure you don't have a conflict of interest as you may be subject to preparer penalties. Make sure you can explain the "ordinary and necessary" reason for each expense, that seems to be a hot button on Sch C audits recently. Good luck! You can do this.

Katheryn makes some good comments but I would add that, if the client goes to the audit, you instruct the client in how to respond. If the agent asks the client a question, tell the client to answer the question only and do not go beyond what is asked. If the question relates to something on the tax return, have the client defer to you as the expert.