(MONEY Magazine) – When you dump four bags of your old clothes on the Salvation Army, workers there will give you a receipt for four bags of old clothes. But the value of the pile as a charitable tax deduction is left up to you to decide. How much ) is too much? The IRS has tried to eliminate most of the guesswork: you should deduct only the current fair market value -- in other words, what the clothes would fetch in a used-clothing store or a flea market. To protect yourself from a challenge, keep records of what you contributed to which charity along with a description of the items. If you claim a deduction exceeding $500, you must file Form 8283, Noncash Charitable Contributions. And if you donate an Imelda Marcos-like shoe collection worth more than $5,000, or other ''similar items'' amounting to that much -- say, a library of books -- you must have a qualified appraiser verify the total using Section B of Form 8283.