I disagree with TaxGuyBill. The charity will almost never attest to the value of the item, and there is nothing in the IRS publication that says they should. What you are required to have is an acknowledgement or receipt from the charity of the donation, as well as your own written records in sufficient detail to be able to determine a deduction value. That generally means you need to know the date (approximately) the item was acquired, how acquired (gift, purchase, etc.), present fair market value, how you determined the value (Craigslist ads, eBay, comparable thrift store prices, etc.) and a description with sufficient detail to determine a value.
*The reason you need to know the date acquired and how acquired is that this may affect the deduction value. Your deduction value is the current FMV, or your cost basis, whichever is lower. For example, if you have a baseball card collection that cost 10 cents per card to acquire 50 years ago, that is your maximum donation value in most cases, even if the value has increased. If you were gifted a painting by the artist that is now worth $10,000, but the artist's basis (for paint and canvass) is $250, that is your maximum donation value. (There is one exception we can discuss if necessary.)
There is nothing that says all this information needs to be on the charity receipt, although it certainly wouldn't hurt to have someone from the charity sign your record. Where people get into trouble with the IRS is when their receipt says "2 bags of clothing" and they have nothing to prove that the items listed in their record match what was actually in the bags.
Your records can be approximate, and it depends on the item -- if you purchased a designer dress for yourself, that now has a Goodwill value of $30, it won't really matter if you bought it 5 years ago for $2000 or 20 years ago for $500, your deduction will always be the FMV which is lower. Where this becomes important is if the FMV today is likely to be more than your cost basis. For example, if you donate an antique chair worth $500 today, it makes a big difference whether your great-grandmother, who bought the chair for $10, gifted it to you while she was alive, or if you inherited it, and the value when you inherited it compared to today's value.
Ultimately, it depends on the mood of the auditor if audited, and the IRS does not have to give you any deduction you can't prove. As a practical matter, they're far more likely to be interested in the antique chair than in 20 pair of used slacks at $4 each.
Now, the situation changes when your donation is more than $5,000 for a single item or "group of similar items." In that case you need a signed appraisal from a qualified appraiser, and the appraiser and a financially responsible person from the charity must sign a form 8283 that you mail to the IRS after e-filing. The definition of a "group of similar items" is a bit nebulous. Donating $5,000 worth of used clothing to Goodwill would probably count, even if donated in many small donations over the year. (The IRS might view that you were splitting the donations on purpose to stay below the limit.) But would $4,000 of used clothing and $4,000 of used furniture, all donated to the same Goodwill, count as 1 group of similar items? I think that would depend on the auditor's mood that day. There may be specific guidance in tax court cases or the auditor's manual but I haven't researched it further.
So you may want to be cautious if you are approaching the $5000 threshold, and think about holding back some items for next year, or being sure you have a good argument and good records for grouping your items into different "groups of similar items" so that each group is less than $5000. The IRS does not disclose their methods of selecting tax returns for audit, but I suspect that playing close to the $5000 threshold may be one item they consider.
