What if I am audited by the IRS but my recordkeeping is poor or my records are lost?

Since I often help clients close out IRS audits when their records are lacking, I thought it was time to answer this question: 
I recieved a letter to meet an auditor from the IRS to audit my 2005 – 2007 taxes. My house was broken into several times, and I don’t have any of my paperwork [...]