Just listen... Like you would (should) with any other person you're having a conversation with. Everyone likes to be listened to, stutterers are no different. Personally, I hate it when people finish words for me if I'm struggling, but I usually shrug it off as that person usually means it well. Also, weirdly enough, many stutterers like to talk about the condition when asked about it. More often than not, we stutter less to the person asking afterwards.