Business mileage is tax-deductable. Business miles accumulate when you drive for the purpose of either doing business or seeking business; going to talk to a potential client, visiting a supplier, and doing something related to the promotional aspects of your business are all considered business mileage. The mileage you drive from your home office to any other location for buisiness pruposes is deductable. As with your other deductions, you must be able to support your claims for business miles. Keep a log by entering your business mileage after each trip. Enter your total daily business mileage on your appointment calendar at the end of each day.
Excerpted from Starting a Home-based Business