Restrictions on Charitable Contributions

All cash contributions made in tax years beginning after August 17, 2006, to any qualified charity must be supported by a dated bank record or a dated receipt.
 
For all contributions you must keep one of the following:
1.  A canceled check or a legible and readable bank account statement that shows:
          a) If payment by check - the check number, amount and date posted and to whom paid,
          b) if payment was by electronic funds transfer - the amount, date posted and to whom paid, or
          c) if payment was charged to a credit card - the amount, transaction date and to whom paid.

2.  A receipt (or a letter or other written communication) from the charitable organization showing the name of the organization, the date of the contribution and the amount of the contribution.
 
Note:  Until August 2006 it was acceptable to keep reliable written records of cash donations that were under $250.

-- From the IRS on "Tax Law Changes for Individuals"