1. Keep an itemized and dated list (I use a spreadsheet for this). As I toss things in boxes/bags, they go on the list and the price is recorded. Without a list, you're only guessing and the IRS frowns on guesses. Even though my receipt shows "X boxes" or in some cases is blank, I have the list showing exactly what was given to match up with the dated receipt.
2. Our local Goodwill keeps a price list on their website showing what they generally sell things for. I use that as my price guide for clothes/household goods/furniture. For books and media that I donate to the library, I use the prices that they sell for when the library has its book sale ($X for hardcover, $Y for paperback, $Z for DVD's).
3. The spreadsheet tallies my totals for the year from each organization and boom, at tax time all I have to do is carry those numbers over to the tax forms.