Treat them as you would anyone. I have two people at work who stutter and I hardly notice it as I'm so used to it.

Be patient with them and try not to make them feel as if their stutter is a bad thing.

Don't jump in and finish/say the word for him/her. Don't appear to be annoyed at how long it might take and don't keep looking at your watch. Therapy is available for those with such problems..of course, the earlier it is received the better it "takes"! :)

Try not to make them nervous, as the condition usually worsens. Let them know it's ok and to take their time. Try not to smile because it can be confused as laugher. Keep a straight and calm face and be patient. Never try to finish their words and it's not polite to repeat what they've just said for your own verification. Make normal conversation because I wouldn't like someone to repeat what I've said either.

Patience, and understanding. I stutter when I get nervous (...or excited), and since I'm already nervous when I'm stuttering, making me more nervous is not going to help. It just takes some time to get out what I'm trying to say, and I will repeat a lot of things. So what? Act like it's nothing.

Smile but not too much...maintain eye contact...take calming breaths if you feel you are getting impatient with them...listen closely and nod...maybe if you are romantic partners, touch your hand to their arm or leg...

i stutter extremely bad. i also tend to make strange faces while stuttering. when I stutter, i become comfortable if the person I am speaking to does finish my sentence, it relieves me of saying it and making a larger fool of myself. never smile while they are trying to speak, unless your close to them because it can easily be confused with laughter. if the stutterer becomes so frustrated that they must stop and it seems they dont want to continue speaking, just tell them you understand what they were trying to say, or tell them it wasnt bothering you and you would like to hear what else they had to say. Never take a deep breathe or sigh while they are trying to speak to you, because it makes you seem impatient or irritated and makes them more uncomfortable increasing their stutter. dont tell them to slow down or breathe because that is also a sign of impatience or irritation. That is all I really have to say, unless you're a teacher, i have plenty to say about that subject. cmt if you are and need advice, because im a freshman and my teachers make me awfully uncomfortable but I am afraid to speak to them about ways they could help, because I dont want to make a fool of myself while speaking. lol.