Enrolled agents have been around since the post-Civil War days and go through relatively grueling training in this very area. Keep good records. It's not enough to just pull your records together year by year on April 15. Get in the habit of keeping good primary and secondary tax records year round and using a personal filing system to keep them with the appropriate tax return. Then, if you're tapped for an audit, the process of preparing is much less stressful.
Primary records are bills and receipts.
Secondary records may be spreadsheets, mileage logs or other summary information you've kept. Warren recommends that you keep all tax returns, but that you keep your backup information for the current year plus three past years. Go back to that year and try to re-create records as accurately as possible. If you've claimed expenses in certain areas, like medical expenses, it's possible that your doctor or hospital will still have those medical records on file. Don't hesitate to call them.