A question I am often asked during tax time is "how much is my donation to (enter your favorite charitable thrift store here) worth?" While it is not a good idea to come up with that amount while you are sitting with your accountant, there are valuation guides provided by the Salvation Army and Goodwill to help you with the numbers. You should list the items you donate, determine the condition they are in, and assess the thrift store value of the items. Attach this to the receipt from the organization and file away in your tax folder. The IRS does not give you a maximum deduction if you do not have receipts.
There are different rules for donations that are greater than $5000. These items must be appraised and documented appropriately.
Donations are deducted on your Schedule A - Itemized Deductions. These deductions will reduce your taxable income if you are able to itemized.