You can write off the fair-market value of anything you donate to a qualifying thrift shop, like Goodwill or the Salvation Army. Fair-market value means the amount the store could sell that item for after you've owned it. So, if I am donating a pair of jeans I bought for 30 dollars a year ago, they won't still be worth that much. Goodwill could sell them for 7-10 dollars if they are still in good shape, so 7-10 dollars would be the fair market value.
Here is a handy sheet Goodwill makes... I use it to help figure out the value of my donations: http://www.goodwill.org/c/document_library/get_file?folderId=102123&name=DLFE-2302.pdf. The number will be higher if the item is in good shape, or lower if it's worn.
Here is an IRS publication on determining the value of donated items: http://www.irs.gov/pub/irs-pdf/p561.pdf
Before you take your donation in, go through everything and write down the item, quantity, and condition, and estimate the free market value. It's up to you to do this, because the person who takes donations at the store usually doesn't. Here is an example of what I typed up for clothes I'm donating to Goodwill tomorrow:
Clothing and Accessories
Halter top – 1 – used $2.00
Long-sleeved t-shirt – 1 –
Short-sleeved t-shirt – 3
Blazer – 1 – nearly new $5.00
Tank tops – 2 – used $2.00
Skirt – 1 – used $4.00
Peacoat – 1 – nearly new $10.00
Wool peacoat – 1 – used, excellent condition $20.00
Dress pants – 4 – used $20.00
Jeans – 2 – nearly new $18.00
Jeans – 1 – used $7.50
Wedge sandals – 1 – nearly
(I don't know why it came out formatted like that, but you get the idea). The I total everything up and put the value at the bottom in a big font. At the top I put what store it was donated to, and the date. I print it out and keep it with my tax papers, along with the receipt I get from the store. I keep careful track in case of being audited, and it's nice to have everything ready when it's time to do my taxes.
When you take everything to the store, all the attendant usually writes down is something like "1 bag of clothes." You have to itemize that for yourself, and you're allowed to determine the value. The receipt will have a place for the value, and the attendant's signature, so make sure both of those are on it. Staple it to the sheet you printed earlier, and you're ready for tax time!
At tax time, you report all your donations on Form 8283: http://www.irs.gov/pub/irs-pdf/f8283.pdf, and http://ftp.irs.gov/pub/irs-pdf/i8283.pdf