If you donate items to charity, keep a folder listing the items (perhaps take a picture of larger items) with amounts to deduct if you itemize your tax deductions. Often, the charity will give you a blank receipt. Attach your list to it.
There are several sites where you can estimate the value of your items:
Goodwill
Salvation Army
Donate Guide
If in doubt use the same process you would use for a yard sale and value the item at 25% of the retail value.