Take pictures showing the desk, file cabinets, computer, and other business equipment.

Keep a sketch of your home to back up calculations of the percentage of the home occupied by the home office.

Print your home address and home office phone number on business cards and stationery.

Keep a log detailing when you use the office, what you work on, and with whom you meet. Notes on a desk calendar will do.

Don't send this documentation with your return. Just keep it with your tax files in case you're contacted by the IRS.