I found the following good advice on a websearch:

Make no assumptions—ask the person what s/he needs.

Be predictable—don't surprise the person.

Let the person with the disorder set the pace for recovery.

Find something positive in every small step towards recovery.

Don't help the person avoid his/her fears.

Maintain your own life so you don't resent the person with the disorder.

Don't panic when the person with the disorder panics, but realize it's natural to be concerned with them.

Be patient and accepting, but don't settle for the affected person being permanently disabled.

Say encouraging words such as: "You can do it no matter how you feel. I am proud of you. Tell me what you need now. Breathe slow and low. Stay in the present. It's not the place that's bothering you, it's the thought. I know that what you are feeling is painful, but it's not dangerous. You are courageous."

Avoid saying things like: "Don't be anxious. Let's see if you can do this. You can fight this. What should we do next? Don't be ridculous. You have to stay. Don't be a coward." These phrases tend to blame the individual for the anxiety.