Tell people about it before it happens. Introduce yourself and say something like "I have a stutter by the way, so if I start tripping over my words, that's why, just so you know. How's your day going so far?" Or, if you know you have problems with a certain word/sound, or if you can feel a block coming, say something like "Oof, I have a stutter and this word is hard for me. Bear with me," and then say it. Being open about it is probably your best bet. You might be the first person a lot of people have come across who stutters, it's not something most people consider. It might not even be something that clicks when they hear it, it could just seem strange to them. Let them know about it so they're not taken aback, so they're more likely to understand.