After 2007, all charitable contributions (no matter how small) need to be substantiated with a written receipt from the organization, a cancelled check, or a bank record or credit card statement showing the organization’s name, the amount, and date or it is not deductible. This includes donations to the Goodwill, the offering plate at church, kids collecting door to door for school, the bell ringers for Salvation Army. For most cases, do not give cash; write a check. A written log maintained by the taxpayer is no longer sufficient. No receipt equals no deduction.
Source: http://www.irs.gov