What documentation is required in order to be reimbursed mileage for business travel using a personal vehicle?

For one-time trips between two primary destinations, an online map source (e.g. MapQuest or Google Maps) is the best source of address-to-address documentation. When traveling to multiple destinations, or travel covering a wide range of dates (for the same business purpose), a log including odometer readings and address-to-address locations are required to document for audit review.