I always use the "I have a medical condition that prevents me from wearing anything that restricts my breathing" excuse.

I had a manager at a fastfood joint ask me if I had any documentation to prove my claim.

I lit into her ass and told her, "According to the ADA and HIPPA acts, its against the law for you to even ask me that question".

She became so frustrated that she just gave up and said, "Uh, what would you like to order?"
Other places I go into, here in NC, most workers are fed up and dont even say anything to people anymore.