I certainly don't know the legalities, but I always understood that you could claim the donation (at whatever value) as soon as it's made. At least that's how it is when you donate to other charities. (Goodwill, etc.)
Not sure how to value cars, however. Clothes and stuff seem to be kind of arbitrary.
As I understand it, there is a maximum value that can be donated to a given entity each year, without documentation.
(Asking Tammy, since she's donated about a dozen bags of clothes this year, already, with more to go. Her advice...Talk to your "tax professional.") Google is probably your friend.