Most of the time, if it isn't something custom designed specifically for the organization I try and donate something relatively neutral like this. Pieces that would appeal to a wide audience.
In an effort to help the charity raise the most funds at the auction, giving something that would be easy to wear by many different types of people can be a good idea. I also like coordinating several pieces. That way the person bidding gets good value for their donation.
If they decide they don't want all of the pieces for themselves they can choose to separate the set and give part away as a gift.