There are two reasons to keep accurate business trip expense records: for tax return deductions and for company reimbursements. For both purposes you must be able to substantiate your business expenses, and the best way to do so is to maintain accurate records while you’re actually on the trip. If you wait until you get home, you risk forgetting expenses or losing receipts and other necessary documentation. How you choose to record your expenses is up to you; PDAs, personal organizers, and handwritten diaries all work just fine. The important thing is to save all sales receipts and any additional information that explains the reason for the expense.
For more on business travel, be sure to read Business Travel Safety Tips.