You can claim anything up to $500 without any receipt or any proof whatsoever, just say I donated $500 worth of clothes each year and nobody will ever ask you anything.
That Said:
You should always get a receipt and you should have at least a basic itemization. You never know what all you will donate later in the year and you’ll kick yourself later if you go over this $500 amount.
If you are donating 6 bags of clothes, you should have a basic list of everything: eg.
2 hoodies – Old Navy
1 Leather Jacket Wilsons Leather
10 pairs jeans – good shape
8 Womens blouses – good shape
Etc. Etc. Etc.
Shouldn’t take more than 10 minutes to itemize bags of clothes.
For more expensive items like an air conditioning unit or a coffee table, especially items that are in excellent shape, take a picture to prove it is in excellent shape.
Always imagine that you are sitting in the audit office and you have to prove your case.
If you donate over $500 in one day, you may have to set a cost basis on the items. Generally the cost basis can just be set to the donation price or higher and you’ll be fine. They’re just making sure you didn’t buy a $5000 car for $5 and you are trying to game the system.
IMHO – Unless you really stretch it, you are not likely to be audited on your charitable donations. Anything within reason will not attract suspicion. And if you have an itemization, you’ll protect yourself.