Listen to the person with the disability. Do not make assumptions about what that person can or cannot do.
When speaking with a person with a disability, talk directly to that person, not through his or her companion. This applies whether the person has a mobility impairment, a mental impairment, is blind or is deaf and uses an interpreter.
Extend common courtesies to people with disabilities as you would anyone else. Shake hands or hand over business cards. If the person cannot shake your hand or grasp your card, they will tell you. Do not be ashamed of your attempt, however.
If the customer has a speech impairment and you are having trouble understanding what he or she is saying, ask the person to repeat rather than pretend you understand. The former is respectful and leads to accurate communication; the latter is belittling and leads to embarrassment.
Offer assistance to a person with a disability, but wait until your offer is accepted before you help.
It is okay to feel nervous or uncomfortable around people with disabilities, and it's okay to admit that. It is human to feel that way at first. When you encounter these situations, think "person" first instead of disability; you will eventually relax.