When the deduction you claim for donated property exceeds $500—in total, not per item—you need to file an extra form with your tax return: Form 8283, Noncash Charitable Contributions. The information required on the form is basically the same as you need to substantiate any charitable gift—what you gave, when and to whom and, for items valued at over $500 each, when and how you acquired the property and your cost or adjusted basis.
(Your basis is the property's value for tax purposes. It's usually what you paid for the property—the cost of stock, including brokerage commissions, for example—but if real estate or other depreciable property is involved, the basis is the cost minus any depreciation claimed.)
Rather than just keeping the information with your records in case you're audited, the IRS demands that you send it in with your return when your write-off for gifts of property exceeds $500.
For more information on deducting non-cash charitable donations, see the Help topic and click on Tax Tips and Advice/Tax Tips/Itemizing Deductions/Noncash Donations.