My oldest started stuttering around three, especially when he was in a hurry or excited. I ended up taking him to a speech therapist for about a year and a half, one time a week. He hasn't had a problem with it since. One of the things we used to do when he would start stuttering was remind him to slow down. We would tell him to take a breath and then tell us what he was saying and he could usually do it fine then.
When ds#2 started stuttering a little I ended up not worrying about it right away because I had gone through it once with his big brother. I figured if it was something he would need therapy for he woul dhave it a while and it would make it difficult for him to communicate things. So far he seems to have grown out of it and he is six, but his stuttering wasn't quite as bad as his brothers.