The best way to prove business travel expenses (including hotels, flights, rental cars, meals, and entertainment) is to use a credit card slip (using your business card, of course) with additional notes on the business purpose. Make the note at the time you incur the expense.

To successfully prove travel expenses for tax deductions, the IRS (and most global tax authorities) requires "contemporaneous" and detailed documentation that proves the trip was ordinary, necessary, and business-related. The gold standard for proof is a travel log or diary combined with original receipts. Your records must clearly show the date, amount, location, and specific business purpose of each expense—for example, a receipt from a hotel must be accompanied by a note of which client you were meeting. For transportation, you should keep boarding passes or train tickets. If you use a personal vehicle, you must track your starting and ending odometer readings to calculate business mileage. Digital tools like QuickBooks, Expensify, or even a detailed spreadsheet are highly recommended for organizing these records. You must retain these documents for at least three years after filing. Crucially, if a trip combines business and pleasure, you must clearly "apportion" the costs; for example, if you spend three days at a conference and two days sightseeing, you can only deduct the airfare and the three days of business-related lodging and meals.