When I sheepishly told my tax preparer that I'd misplaced my receipts for the donations I made to Goodwill throughout 2008, he gave me a good-willed evil eye and then offered a great piece of advice. He said that while he's claimed thousands and thousands of dollars in donations throughout the years, and never once has anyone come knocking for proof, but that not everyone is left unbothered.
He picked up a smart tactic from his clients, and I'm going to start using it the next time I have a donation bag ready to go: Take pictures of the items you donate and then upload the photos to your computer. Should anyone come asking for evidence, you have more than a receipt to support your story. When it comes to taxes, it's always better to be safe than sorry.