In general, when you meet a person with an assistance dog, please remember that the dog is working, even if it doesn't appear that way to you!

You should be careful not to do anything that will distract or interrupt the dog from performing its job.

Some specifics:
Speak to the person first.
Do not make distracting noises aimed at the assistance dog...such as whistling or "kissing" at it.
Do not touch the assistance dog without asking permission.
Do not feed the assistance dog.
Do not ask personal questions about the handler's disability or intrude on his or her privacy. You would expect the same from them.
Don't be offended if the handler declines to chat about the assistance dog. Remember, they may be in as much a hurry as we all are in our hectic lives.

Magic: The magic of the smiles that cross the troubled faces when training the dogs out in public"