We took photos of the items, and collected (blank actually) receipts from the charity. If audited, we'll fill them out with the lists we already have. We then input our donations into "itsdeductible" to get a value of items based on average online selling prices. I personally err on the side of caution and 1) input everything as "medium" value unless it's actually still new with tags (luckily not many!), 2) only put on my taxes 80 or 90% of what we actually have. I've heard that if you are audited, they actually need to ensure it's right, so if you underput things, you should get money back from them. I may not fully maximize my deductions, but I get peace of mind this way. And if I'm ever audited, I have some leeway if they challenge my donations (as say, being below good condition) so I wouldn't pay penalties, with the bonus for putting up with it that I should get that 10-20% back.
Note - if you are giving in a single item worth over a certain amount, I think you need an appraisal for it.
ETA -
https://itsdeductibleonline.intuit.com/help/donationsTrackerHelpMain.html (free to enter information but I highly recommend printing a report - it unfortunately deleted information I entered for last years return)
"ItsDeductible generates item values using a blend of eBay sales data and thrift store pricing. To do this, we collect the data throughout the year and provide updated item values - known as "established values" - at the end of the year. This ensures that the item values provided on your tax form accurately represent the fair market values of the items when they were donated."