I use a free program called "It's Deductible" available online at:
You can create and save a list of all the things you donate to charity and directly import it into Turbotax if you use that software. Otherwise just printout the list.
I was amazed at how much the items I donated were worth and how easy this was to do. I know that Goodwill has something similar to help you value your donations, but this was very easy to use.