What records should I keep to prove the business use of my automobile ?

The best source document to use is a calendar diary. Enter the odometer readings at the beginning and at the end of each trip. Enter any actual auto expenses incured on the trip. At the end of the year you may be able to optimize between the higher of the actual or mileage rate expenses. Enter the names of the company or the people you met with. Enter the business purpose of the trip. Enter any meal or entertainment and other miscellaneous expenses incurred in addition to using your car.