Haddayr Copley-Woods has an awesome and entertaining piece on Minnesota Public Radio about how to interact with people with disabilities.
(That page provides both audio and a transcript.)
My favorite bits:
DON'T 'Help' without asking. If someone in a wheelchair has rolled up to a door, they probably have some sort of a plan.
DON'T Ask strangers intrusive personal questions such as "what's wrong with you." Sure, you are curious. I myself find some fashion choices curious, but I have learned to live with mystery.