Characters will not sign anything that is being worn.
I did an autograph shirt for me when I went this summer. Shirts were much more difficult for the characters to sign. What we did was bring a clipboard (full size) and stretch the shirt over it. We then secured the shirt with binder clips so it was wrapped tighter and didn't give as much when the characters signed. It was easier for the face characters than it was for the others and we often still had to help hold it tight enough for them to sign with the sharpie we brought. Most of them seemed to have enough practice signing that they were able to compensate for the difficulty of signing on fabric and the signatures came out great.