1. Be Patient
Most people who stammer strongly prefer to speak for themselves. You may be tempted to finish a person's sentences or 'fill in' words but this does not help.
Remember that it is OK to stammer. Don't give advice such as: 'slow down', 'take a breath', or 'relax'. Maintain natural eye contact, listen, and wait patiently until the person has finished speaking.
2. Be a Good Listener
Let the speaker know, by what you say and do, that you are listening. Try to actively convey a relaxed and accepting attitude as any obvious discomfort that you show will only increase the discomfort of the person who stammers. Focus on what the person is saying, not how they are saying it.
3. Remember That Stammering Varies
People who stammer can have most difficulty when starting to speak and less difficulty once underway. Don't be surprised if a person stammers more in some situations than others. The telephone, speaking in front of a queue or in earshot of others can cause increased difficulties.
4. Remember that stammering is not caused by Nervousness.
While a speaker may appear nervous, keep in mind, that nervousness is a result of embarrassment about their stammering and the environment rather cause by it
If you are Not Sure How to Respond, Ask the Speaker - but always do this sensitively and in a way that leaves the Speaker in Control.
This might involve asking an open question such as, "Is there anything I can do to make this easier for you?" Or, if someone is stammering severely, closed questions such as "Would you prefer to go somewhere quieter?" or "Would you prefer to write this down?"
Please note that the tone of these questions is very important. Bear in mind, too, that some speakers may be uncomfortable talking about their speech, but many would welcome your respectful interest.
In conclusion..
Try to empower the person by offering a choice rather than imposing your solution. Always be on the side of being patient and giving the person the opportunity to speak for him/herself.