These tips are from The Stuttering Foundation of America:
"Stuttering may look like an easy problem that can be solved with some simple advice, but for many adults, it can be a chronic life-long disorder. Here are some ways that you, the listener, can help.
1. Don’t make remarks like: “Slow down,” “Take a breath,” or “Relax.” Such simplistic advice can be felt as demeaning and is not helpful.
2. Let the person know by your manner that you are listening to what he or she says — not how they say it.
3. Maintain natural eye contact and wait patiently and naturally until the person is finished.
4. You may be tempted to finish sentences or fill in words. Try not to. Use a relatively relaxed rate in your own speech — but not so slow as to sound unnatural. This promotes good communication.
5. Be aware that those who stutter usually have more trouble controlling their speech on the telephone. Please be patient in this situation. If you pick up the phone and hear nothing, be sure it is not a person who stutters trying to start the conversation before you hang up.
6. Speak in an unhurried way — but not so slowly as to sound unnatural. This promotes good communication with everyone.