The law disallows an otherwise allowable deduction for any expense for traveling (including meals and lodging), entertainment, gifts, or listed property (e.g. passenger vehicles, computers) unless used exclusively at your place of business, unless the expense is substantiated by adequate records or by sufficient evidence corroborating your own statements.
In addition, the regulations generally require you to maintain documentary evidence (such as receipts, paid bills, etc.) for:
1) Any lodging expenditure.
2) Any other expenditure of $75 or more.
For business travel, the documentation should include the amount, date, place, and business purpose of the travel.
For business entertainment expenses, the documentation should include the amount, date, place, and business purpose of the entertainment as well as the business relationship of the person(s) entertained.
For business gifts, the documentation should include the amount, date, description of gift, business purpose of gift, and business relationship of recipient of the gift.
For listed property, the documentation should include the amount (e.g. cost), business or investment use based on mileage, etc., date of the expenditure, and business or investment purpose of the property.